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SPONGES

Online Encyclopedia
Originally appearing in Volume V25, Page 732 of the 1911 Encyclopedia Britannica.
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SPONGES. The Sponges or Porifera form a somewhat isolated phylum (or principal subdivision) of the animal king-dom. This phylum includes an immense number of marine and fresh-water organisms, all of which agree amongst them-selves in possessing a combination of important structural characters which is not found in any other animals. Though the phylum is a very large one yet almost the only examples with which the name " sponge " is popularly associated are the common bath sponges (species of the genera Euspongia and Hippospongia), which are amongst the most highly organized and least typical members of the group. The history of the group begins with Aristotle, who recognized several different kinds of sponge, some of which were used by the Greek warriors for padding their helmets. Owing, however, to the permanently fixed character, irregular growth and feeble power of movement in the adult organism, it was not until the advent of microscopical research that it was definitely proved that the sponges are animals and not plants. Indeed our scientific knowledge of the group can scarcely be said to begin much before the middle of the loth century, when the classical researches of R. E. Grant, J. E. Gray, H. J. Carter and J. S. Bowerbank laid the foundations of modern spongology. It very soon became evident that the group is one which illustrates with remarkable clearness and beauty those laws of organic evolution which were beginning to attract so much attention from zoologists, a fact which found abundant recognition in Ernst Haeckel's epoch-making work on the Calcareous Sponges published in 1872. This was followed by a series of remark-able researches by F. E. Schulze on the minute anatomy, histology and embryology of the group, which have served as a pattern to all subsequent investigators. In more recent years our knowledge of the sponges has advanced very rapidly, especially as the result of the great series of scientific exploring expeditions inaugurated by the voyage of H.M.S. " Challenger." The large collection made by the " Challenger " expedition alone, necessitated a complete reorganization of our systematic knowledge of the phylum, and afforded the foundation upon which our present system of classification has been built up. There is perhaps no great group of the animal kingdom in the study of which greater advance has been made in the last twenty years. It is impossible in the space at our disposal to do justice to the numerous valuable memoirs which have appeared during this period, but reference to the more important works of recent investigators will be found in the bibliography at the end of this article, while for a comprehensive account of the whole subject the reader should refer especially to Professor E. A. Minchin's article in Sir E. Ray Lankester's Treatise on Zoology. General Characters of the Phylum.—The sponges are all aquatic organisms, and for the most part marine. They vary in size from minute solitary individuals, scarcely visible to the naked eye, up to great compound masses several feet in circumference, and in form from almost complete shapelessness to the most exquisite and perfect symmetry. The indefiniteness of shape and size which characterizes the vast majority of the group is due to the power of budding, which is almost universal amongst them, whereby extremely complex colonies are built up in which it is usually impossible to determine the limits of the individual zooids or persons, while very frequently, by a process of integration, individuals of a higher order are produced which again form colonies by budding (fig. 2). The entire body of the sponge is penetrated by a more or less complicated canal-system, beginning with numerous inhalant pores, scattered over the general surface or collected in special pore-areas, and ending in one or several larger apertures, the vents or oscula, situated usually on the uppermost portions of the sponge (fig. 8). If the living animal he kept underobservation it will be seen that a stream of water is ejected with considerable force from the vents, carrying with it minute particles in suspension. At the same time numerous smaller streams enter the canal system through the inhalant pores, bringing with them the minute particles of organic matter upon which the sponge feeds and the oxygen which it requires for respiration. This stream of water may be temporarily interrupted by the closure of the pores and vents, to be resumed apparently at will. It is maintained by the activity of certain cells, known as collared cells or choanocytes (fig. 35, g, fig. 36), which line the walls of the canal system either throughout their entire extent or in certain regions only. These cells bear an extraordinarily close resemblance to the choanoflagellate Protozoa or collared Monads. Each is provided with a filmy protoplasmic collar and a long whip-like flagellum, and the movements of the latter drive the water out of the canal-system through the vents and thus keep up the circulation. In all but the simplest sponges the collared cells are confined to certain portions cf the canal system known as flagellated chambers (fig. o), the size, form and arrangement of which vary greatly in different types. That part of the canal-system which is not lined by collared cells is covered with a flattened pavement-epithelium (fig. 34, I), and so also is the outer surface of the sponge. The space between the various branches of the canal-system is occupied by a gelatinous ground-substance (mesogloea) in which amoeboid and connective-tissue cells are em-bedded (fig. 34, 3, 4, 5; fig. 35, a), and in which in most cases a well-developed skeleton is secreted by special cells known as scleroblasts. This skeleton (figs. 24-32, &c.) supports the extremely soft tissues of which the body is composed, and consists either of mineral spicules (carbonate of lime or silica) or of horny fibres (spongin), or of a combination of siliceous spicules with spongin. In many cases the proper skeleton is more or less completely replaced by sand. The question as to how far the cell-layers of the sponge body correspond to the " germinal layers " usually recognizable in other multicellular animals is an extremely difficult one and not yet by any means settled. It has until recently been generally sup-posed that the flattened epithelium which covers the outer surface of the sponge, together with part of that which lines the canal-system, is ectodermal, while the collared cells and the remainder of the flattened epithelium lining the canal-system are endodermal, and the term mesoderm has been frequently applied to the middle gelatinous layer. Recent embryological research, however, makes it extremely doubtful whether this view is justifiable, and whether indeed the germ-layers of typical Metazoa can be identified at all in the Porifera. Embryological research, moreover, tends to show that the primitive gastral epithelium (of collared cells) is in most sponges completely replaced, except in the flagellated chambers, by an invasion of the dermal epithelium (composed of flat pavement, cells). Sexual reproduction, by means of ova and spermatozoa, is probably universal throughout the group. The segmentation of the ovum gives rise to the free-swimming ciliated larva (figs. 38, e, 39) in the form of a hollow " amphiblastula " or of a solid " parenchymula." This larva becomes attached and, by means of a more or less complex metamorphosis, gives rise to the young sponge. During the metamorphosis the outer, ciliated or flagellated cells of the larva take up their position in the interior of the body and give rise to the collared cells of the adult; while the inner cells (of the parenchymula) migrate outwards and form the superficial epithelium, so that the position of the so-called " ectoderm " and " endoderm " is completely reversed in the adult as compared with the larva. A sexual reproduction is effected by budding, and the buds may either remain attached to the parent and form colonies or become detached and form entirely separate individuals. Types of Structure.—We may illustrate our account of the general characters of the group by a brief description of the anatomy of three widely divergent types, selected as being fairly representative of the entire group, viz. Leucosolenia, Plakina and Euspongia. Leucosolenia.—The genus Leucosolenia includes a number of calcareous sponges of very simple structure, and thus forms a suitable starting-point for our studies. Imagine a minute, thin-walled sac (fig. I), attached at the lower end to some rock or seaweed, and enclosing a spacious cavity in its interior. This cavity is the gastral or digestive cavity, and it opens to the exterior 716 through a wide vent or osculum at the upper extremity of the sponge. The thin wall is also pierced by numerous small inhalant pores or prosopyles. The inhalant pores, the gastral cavity and the vent constitute the canal-system, through which a stream of water can be kept flowing by the activity of the collared cells which line practically the whole of the gastral cavity. Each collared cell consists of an oval nucleated body surmounted by a filmy proto- plasmic collar, in the middle of which the whip-like flagellum projects into the water. They are placed close together, side by side, and thus form a continuous layer, extending almost up to the vent and interrupted only by the inhalant pores. The outer surface of the sponge is covered by a single layer of flattened pavement-epithelium or epidermis. Some of these cells, distinguished as porocytes, become perforated by the inhalant pores, around which they form contractile diaphragms capable of opening and closing, and thus regulating the supply of water. Between the outer protective, dermal epithelium, and the inner gastral epithelium of collared cells, lies the mesogloea, a layer of gelatinous material containing cells of at least two kinds, amoebocytes and scleroblasts. The former closely resemble the amoeboid white blood corpuscles, or leucocytes, of higher animals, and have the power of wandering about from place to place in the sponge-wall. They probably serve to distribute food material and carry away waste products, and some of them undoubtedly give rise to the ova and spermatozoa. The scleroblasts are derived from cells of the dermal epithelium which migrate inwards into the gela-(AfterHaeckel.) tinous ground-substance and there secrete the FIG. I.—Leucoso- spicules of which the skeleton is composed. lenia primordial is These spicules are composed of transparent (Olynthus form). crystalline carbonate of lime (calcite), and may be of three fundamental forms: triradiate, quadriradiate and monaxon. It has been shown by E. A. Minchin, however, that the triradiate and quadriradiate types are not simple spicules but spicule-systems, each formed of three or four primary spicules, originating from as many mother-cells and only secondarily united. In fig. I only triradiate spicules are represented, but very often all three kinds are present in the same sponge (cf. fig. 24). The triradiates lie in the mesogloea with their three rays extended in a plane parallel to the surfaces of the sponge-wall, and form a kind of loose scaffolding upon which the soft tissues are supported. The quadriradiates resemble the triradiates in form and position, but a fourth ray is developed which projects through the layer of collared cells into the gastral cavity, where it serves as a defence against internal parasites. The monaxon spicules have one end embedded in the mesogloea while the other projects outwards and upwards and serves as a defence against external foes. Although all species of the genus Leucosolenia agree essentially in structure, yet they exhibit very great diversity in external form. This is due to the habit of budding and colony formation. All start life after the metamorphosis of the larva in the simple sac-shaped condition which we have just described, and to which the name " Olynthus-type " is sometimes applied. This is indeed the simplest type of sponge organization known to us and we must look upon the Olynthus as representing a primary sponge-individual or " person." By a simple process of budding, in which (After Minchin, from Lankester's Treatise on Zoology.) use, 'Osculum. plc, Sphincter of osculum. cl. osc, Closed osculum. diz, Diverticula. contr. osc, Closed oscula in con- osc. div, Diverticula from which tracted part of colony. new oscula arise.the buds all remain united together by their bases, we get a branched colony in which the persons or zooids are still easily recognizable, each with its own vent or osculum. Very frequently, however, the zooids become elongated into slender cylindrical tubes which branch in an extremely complex manner and anastomose with one another in many places to form networks, in which it is no longer possible to recognize the component individuals (fig. 2). This is known as the " Clathrina " type of structure, and we may look upon a Clathrina colony as an individual of a higher order, which may assume a definite external form and even acquire a secondary internal cavity (pseudogaster), opening to the exterior through a secondary vent (pseudosculum), while the outer tubes of the colony may give rise to a protective skin (pseudoderm), perforated by secondary inhalant pores (pseudopores) which are obviously quite distinct in nature from the primary inhalant pores or prosopyles of the Olynthus. Other types of colony-formation in the genus Leucosolenia will be discussed when we come to deal with the canal-system in general. Plakina.--The genus Plakina includes some of the simplest of the siliceous sponges. Just as in the Calcarea the most primitive " person " or individual is represented by the Olynthus type, so in the non-calcareous sponges we may recognize a primitive or (A ter Keller.) fundamental form of individual to which the name "Rhagon " has been applied. This is the first stage reached after the meta-morphosis of the larva in certain species, and the little sponge consists of a cushion-shaped sac, attached below by a broad flattened base and terminating above in a single vent or osculum (fig. 3). There is a large gastral cavity lined by pavement-epithelium and surrounded by a number of more or less spherical " flagellated chambers," lined by collared cells. These chambers open into the gastral cavity by wide mouths (apopyles) and communicate with the exterior by smaller inhalant pores. The entire outer surface of the sponge is covered with pavement-epithelium and there is a well-developed mesogloea which may contain spicules. This Rhagon may be compared to an Olynthus which has become flattened out from above downwards and from which a number of small buds (the flagellated chambers) have been given off all round, except from the attached basal portion; so that the whole forms a small colony, in which the collared cells have become restricted to the buds. We may, therefore, perhaps, look upon the Rhagon as an individual or person of a higher order than the Olynthus. Like the Olynthus the Rhagon occurs as a transient stage in the development of certain sponges, but we do not know any non-calcareous sponge which remains in such a simple condition through-out life. In Plakina monolopha, for example, the entire *all of the Rhagon becomes thrown into folds (fig. 4) so that a system of inhalant and exhalant canals is formed between the folds, through which the water has to pass on its way to and from the chambers. The inhalant canals lead down between the folds from the outer surface of the aponge. In P. monolopha they are wide and ill defined. In another species, Plakina dilopha, they become constricted to form perfectly definite, narrow canals, by the development of a thick layer of mesogloea (and pavement-epithelium) which covers the outer surface of the sponge in such a manner that the folded character is no longer visible externally. The external openings of the inhalant canals now form definite dermal pores. In such a sponge as this the folded chamber-toyer of the sponge-wall is sometimes called the choanosome, while the external layer of mesogloea and pavement-epithelium is called the ectosome. In a third species, Plakina trilopha, further folding of the " choanosomal lamella " takes place and we thus get a still more complex canal-system. In Plakina the spicules are composed of colloidal silica. The fundamental spicule form is the primitive tetract or calthrops, consisting of four sharp-pointed rays diverging at equal angles from a common centre (fig. 5, a–e). Modifications of this form occur in two directions: in the first place some of the tetracts, by branching of one ray, give rise to " candelabra," while others, by suppression of rays, give rise to forms with three or even two rays only, triacts and diacts, the latter sometimes termed oxeate (fig. 5, f–l). The arrangement of the spicules is very irregular; the candelabra alone are definitely arranged (at the surface of the sponge). the other forms are thickly scattered without any sort of order throughout the mesogloea. Euspongia.—The genus Euspongia, to which belong all the finer bath sponges, is a typical example of the true " horny " sponges or Euceratosa, characterized especially by the fact that the skeleton is not composed of spicules but of so called horny fibres, A living Pf ' c. (After F. E. Schulze. From a coloured plate in Zeits. fur Wissen. Zoologie. by permission of Wilhelm Engelmann.) p.f, Primary fibre of skeleton. i.c, Inhalant canals. s.f, Secondary fibres. e.c, Exhalant canals. d.p, Dermal pores (inhalant). f.c, Flagellated chambers. fibres, connect the primaries in all directions and themselves branch and anastomose freely. The primary fibres contain particles of sand or foreign spicules which are taken in by their growing SPONGES 717 bath sponge appears as a dark-coloured, irregular or sometimes I speak of under the name " sponge." It consists of a very close cup-shaped mass attached by the under surface to the sea-bottom. network of spongin fibres (closely resembling silk in chemical The outer surface is covered by a skin or dermal membrane, elevated composition), some of which, known as primaries, run towards the in innumerable minute conuli by the growing apices of the primary surface at fairly regular intervals, while others, known as secondary a, Ciliated embryo (the central e, Rhagon stage, viewed as a transparent object, showing the inhalant pores on the surface and the flagellated chambers in the interior; the osculum is not shown. Part of vertical section through adult sponge, showing the folded choanosomal lamella or spongophare. embryo. ov, Ova. bl, Embryo. skeleton fibres. This skin is pierced by a vast number of inhalant dermal pores of microscopic size, and by a much smaller number of comparatively large vents or oscula. When the sponge is removed from the water the soft tissues rapidly decay and leave behind only the elastic " horny " skeleten, which is what we usually (After F. E. Schulze. From a plate in Zeitschrift fur Wissen. Zoologie, by permission of Wilhelm Engelmann.) Spicules, a—e, tetracts or calthrops; f—k, triacts or triradiates; i—t, diacts, showing how the monaxon form (I) may be derived from the primitive tetract (a) by suppression of actines.car (After F. E. Schulze. From Lankester's Treatise on Zoology.) surrounded by spongoblasts. sp.f, Spongin fibre; sp.bl, Spongoblasts. Coll, Collencytes. apices at the surface of the sponge, and the presence of which may greatly injure the quality of the sponge. The connecting fibres are only about 0.035 mm. in diameter, or even less, and the primaries are a little thicker, while the meshes between the fibres are so narrow as to permit of the soaking up of water by capillary attraction, (After F. E. Schulze.) Flo. 8.—Euspongia officinalis (bath sponge). Diagram of the arrangement of the canal-system as seen in vertical sections of two young individuals. d.p, Dermal pores; o, Oscula; r, Rock to which the sponges are attached. part should be shaded). b, Part of section of ciliated embryo. col, Inner cell-mass. ec, External, columnar cells. fl, Flagella. c, Attached embryo, viewed from above, with the gastral cavity appearing in the interior. d, Vertical section of attached f, the property upon which the economic value of the bath sponge depends. In the living sponge the fibres are embedded in the mesogloea, where they are secreted by special cells known as spongo- blasts, which are often found thickly clustering around them (fig. ;). The canal-system (figs. 6, 8) is very complex and shows but little indication of its origin from a folded rhagon. The in- halant pores lead each into a short, narrow, inhalant canal; these unite in roomy subdermal cavities lying in the ectosome, and from these in turn the main inhalant canals come off. The latter divide and subdivide, and thus ramify through the deeper parts of the sponge amongst the flagellated chambers, to each of which a small number of slender canaliculi are ultimately given off (fig. 9). The chambers themselves, lined by the usual collared cells, are small and approximately spherical, and each one discharges its water through a short and narrow exhalant canali- culus (g. 9). The openings of the inhalant canaliculi into the chambers, of which there are several, correspond to the prosopyles of an Olynthus, while the single exhalant opening, or apopyle, may possibly (After F. E. Schulze. From a plate correspond to an Olynthus osculum. in Zeits.far Nissen. zoologie, by per. The exhalant canaliculi unite to-mission of Wilhelm Engelmaun.) gether to form larger and larger parts of the canal-system, contains cells of various kinds, embedded in a very granular matrix. Comparative Anatomy. external Characters.—Amongst the simpler calcareous sponges, which are all of comparatively small size, the external form is usually symmetrical and is evidently a kind of outward expression of the arrangement of the canal-system. This is well seen in the simplest form of all, the sac-shaped Olynthus, and also in its simpler Syconoid and Leuconoid derivatives (described later on), which may be regarded either as individuals of a higher order or as colonies of Olynthus persons grouped around a central individual whose large gastral cavity opens to the exterior through the single osculum. In the more complex Leuconoids, however, the process of colony formation becomes very irregular and may give rise to great compound masses, with many vents. In these masses we may perhaps recognize the presence of individuals of three orders: (1) the primitive Olynthus persons, represented by the individual flagellated chambers; (2) the Leuconoid persons, indicated each by its osculum; and (3) the entire colony formed by the union of many such Leuconoid persons in an irregular manner. It is, however, very doubtful how far the flagellated chambers in such forms as this can be regarded as morphologically equivalent to Olynthus persons. In the non-calcareous sponges we are always dealing with individuals of a high order, which usually form complex aggre- gates (colonies) of large size and very various shape. As a general rule the form of those non-calcareous sponges which grow in shallow water is extremely irregular and variable while at great ocean depths the shape is usually defi- nite, constant and often exquisitely symmetrical, a fact which may perhaps (After Ridley and Dendy. From be accounted for in part by the absence a plate in .. Challenger" Reports, of disturbing influences such as are met xx., by permission of the Controller with in shallow water. Perhaps the of H.M. Stationery Office.) most extraordinary external form yet (After Ridley and Dendy. From "Challenger" Reports, xx., by permission of the Controller of H. M. Stationery Office.) as an adaptation to the special exigencies of the environment. Thus, for example, many species are provided with long stalks which lift up the body of the sponge out of the soft ooze in which it would otherwise be smothered, while the bottom of the stalk is frequently extended in root-like processes which serve to attach it to some solid object (e.g. Stylocordyla). In other cases the sponge supports itself on the surface of the ooze by long stiff processes, formed of bundles of spicules which radiate from the central, cap-shaped body ; this is known as the " Crinorhiza form," and is met with in several distinct genera (fig. II). Amongst the Hexactinellida, which are essentially a deep-water group, many very beautiful external forms are met with, the best known, perhaps, being the so-called Venus's flower basket (Euplectella, fig. 12). Flabellate (or fan-shaped) and cup-shaped forms are frequently met with even amongst shallow-water sponges, and in widely separated genera, such as Poterion (the great Neptune's cup sponge) and Reniera testudinaria. In Phyllospongia the flabellate and cup-shaped forms pass insensibly into one another, the cup being apparently merely a folded lamella. Slender branching forms are also not uncommon in shallow water, as seen in the common Chalina oculata of the British coast. Spherical forms, such as Tethya, likewise occur. By far the greater number of shallow-water sponges, however, are quite irregular in shape and either form crusts of varying thickness on the surface of rocks and sea-weed, or large and massive aggregates which may rise to a consider-able height above the substratum. In the boring sponges (Family Clionidae) the sponge occupies an elaborate system of chambers and passages which it excavates for itself in the shells of Mollusca and other calcareous organisms. The common British Cliona celata begins (After F. E. Schulze. From a plate in "Challenger" Reports, xxi., by permission of the Controller of H.M. Stationery Office.) life in this way, but soon outgrows the housing capacity of its host, whose shell then serves merely as a base of attachment for the large independent sponge-colony. One of the most striking features of living sponges is their colour, which is often very brilliant. Yellow, red, orange, purple, brown, black, green and blue are all met with, in varying degrees of purity and intensity, amongst the commoner Non-calcarea; whilst the calcareous sponges are usually white. It appears probable that the colour is more or less constant for each species, and may therefore afford a useful guide to specific identification. As a rule the colour is lost is spirit-preserved or dry specimens, but a noteworthy exception is found in the brilliant purple Suberites wilsoni of Port Phillip, in which the colour, though soluble in water, is permanent in dry specimens and in alcohol. The colouring matter is some-times lodged in special pigment cells belonging to the sponge itself, and sometimes in symbiotic algae, with which the mesogloea is frequently filled. Canal-system.—Whether we start with the primitive Olynthus form of the Calcarea or with the more advanced Rhagon of many Non-calcarea, it is evident that further advance in the complication of the canal-system is arrived at either by budding or folding, or by a combination of these processes. As, however, the canal-systems of the calcareous and of the main types of non-calcareous sponges have been evolved along perfectly independent lines it will be well to consider them separately. In the genus Leucosolenia (Calcarea Homocoela) the primitive Olynthus form may, as we have already seen, give rise, by branching and anastomosing, to complex reticulate colonies of the Clathrina type, in which a pseudoderm, pierced by inhalant pores, may cover over a system of inhalant canals which are simply the inter-spaces between the branching tubes of which the colony is made up, while at the same time a centrally placed pseudogaster, which is simply a space enclosed by upgrowth of the colony around it, may form the main exhalant canal and open to the exterior through a well-defined vent or pseudosculum. In this direction perhaps the most remarkable modification arrived at is that of Leucosolenia cavata, in which the Clathrina tubes, lined by collared cells, widen out into large irregular spaces, while the inhalant interspaces become constricted into narrow canals lined by collared cells on the outside. We have here a kind of inversion of the ordinary Clathrina canal-system, but a perfectly gradual transition from the ordinary to the inverted condition is seen as we pass from the older to the younger parts of the colony. In Leucosolenia (Dendya) tripodifera (fig. 13) we find a totally different type of colony formation, which is of great importance as indicating in its canal-system the possible starting-point of a line of evolution which culminates in the highest Calcarea. I-Iere a large central individual, whose spacious gastral cavity is lined by collared. cells, gives off radial buds from all sides, which branch slightly and terminate in blind ends in contact with one another, so that the entire colony has an approximately even surface. The inhalant canals are represented by the interspaces between the radial tubes, between the blind extremities of which the water finds its way in from the outside. There is only a single vent, situate at the extremity of the central cavity. This cavity must be regarded as the original gastral cavity of a parent Olynthus, from which the radial tubes have been produced by budding. (After Dendy. Simplified from a coloured We have next, amongst the plate in Trans. Roy. Soc. of Victoria, Calcarea Heterocoela, the Sycon elboume, vol. in. pt. I.) Olynthus. In the simplest Syconoid forms (Sycetta) the radial chambers remain perfectly straight and unbranched. They do not touch one another at all and there is no trace of an ectosome or dermal cortex, and hence there are no true inhalant canals, and the water circulates without interruption between the chambers. In the genus Sycon (fig. 14) the walls of adjacent chambers come into contact with one another and fuse together and thus give rise to more or less well-defined inhalant " inter-canals." The chambers themselves may branch, and in some species of Sycon a thin, pore-bearing dermal membrane connects together their distal extremities and covers over the entrances to the inhalant canals. The canal-system now exhibits all the different parts found in the most highly-organized sponges: viz. dermal pores, inhalant canals, flagellated chambers, exhalant canal and osculum. In the genus Grantia and its allies (e.g. Ute, fig. 15) pros, the thin dermal membrane c.g.c, of Sycon is converted into a i.c, well-developed cortex, cover- g.cor, big the extremities of both g.q the inhalant canals and the radial chambers, and some- s.g.s, times containing a system of special cortical inhalant canals. We may now distinguish between an ectosome t.ox, (the dermal cortex), which contains no flagellate cham- bers, and a choanosome in which chambers are present. The next stage has probably been arrived at by a kind of folding of the choanosome, for we find the chambers arranged (After Polejaeff.) radially, not around the central gastral cavity but around diverticula of the latter which form special exhalant canals. This condition, sometimes called the " sylleibid " type, is not characteristic of any particular genus 'or family, but occurs in a few isolated species, such as Leucilla connexiva (fig. 16). A somewhat similar condition may be arrived at by branching of the radial flagellated chambers, as in Heteropegma (fig. i7). The next stage is marked by great reduction in the size of the chambers, which may become almost spherical, and by further folding cf the choanosome, so that in a section of the sponge-wall we see the small chambers scattered irregularly in the mesogloea between the numerous branches of complicated inhalant and exhalant canals. Each (From Dendy, in Quart. Journ. Micro. Sc., new series, xxxv., by permission of J. and A. Churchill.) fl.ch, Flagellated chamber. ex.op, Its exhalant opening or apopyle. Prosopyle. Central gastral cavity. Inhalant canal. Gastral cortex. Gastral quadriradiate spicule. Subgastral sagittal triradiate spicules, forming the first joint of the articulate tubar skeleton. Tufts of monaxon spicules at the ends of the chambers. chamber still has several prosopyles, through which it receives water from the ultimate branches of the inhalant canals, while it opens into a relatively large exhalant canal by a wide apopyle. This is the highest type of canal-system met with amongst the Calcarea. It is sometimes known as the Leucon type and is seen in most species of the genus Leucandra, as well as in many others. (After Polejaeff.) It is almost identical with one of the types commonly found in non-calcareous sponges (e.g. Plakina, fig. 4), but has of course been evolved independently. The various types of canal-system met with in the Calcarea are connected together by numerous intermediate forms, thus forming a very interesting evolutionary series, while both the Sylleibid and Leuconoid types appear to have been in-dependently evolved several times, thus affording excellent examples of the phenomenon of convergence, a phenomenon which is very frequently me,; with amongst sponges. (After Polejaeff.) In describing the anatomy of Plakina as a type of non-calcareous sponge, we have traced the development of a fairly complex canal-system from the so-called Rhagon form. We can, however, hardly regard the Rhagon as representing a fundamental type of canal-system common to all the Nen-calcarea, for in some of the Myxospongida, which are the most primitive of all, and again in the Hexactineliida, we find a type characterized by the presence of elongated sac-shaped flagellated chambers resembling those of the Syron type amongst the Calcarea, and these chambers are arranged radially around the exhalant canals (Halisarca, Hexactinellida). The first recognizable stage in the evolution of the canal-system of the Non-calcarea would thus appear to be a condition not unlike that of Sycon, with a number of elongated chambers arranged radially around a central gastrzl cavity and having their blind outer extremities covered over by a dermal membrane. This stage is very nearly reproduced in the young form of a Hexactinellid sponge, Lanuginella pupa. From some such form the Rhagon type may perhaps be derived by flattening out of the lower end of the sponge into a broad base of attachment, and by reduction in the size of the flagellated chambers, accompanied by a more irregular arrangement. Starting from the primitive Myxosponge ancestor, with large sac-shaped chambers, radially arranged, the Non-calcarea have apparently developed along four main lines, giving rise to the existing Myxospongida, the Hexactinellida (Triaxonida), the Tetraxonida "Sc. (After F. E. Schulze. From Lankester's Treatise on Zoology.) specimen (spicules omitted). d.m, Dermal membrane. g.rrt, Gastral membrane. sd.tr, Subdermaltrabecularlayer. G.C, Gastral cavity. fl.c, Flagellated chamber. osc, Region of future osculum. sg.tr, Subgastral trabecular layer. and the Euceratosa. The Myxospongida have retained the large size of the chambers in certain forms (Halisarca, Bajalus), but have lost this primitive character in the more advanced members of the group (Oscarella). The Hexactinellida have retained the large size and radial arrangement of the flagellated chambers throughout their entire series. The chamber layer, however, tends to become more or less folded (fig. 19), and always lies between two layers of !Lc OR o t1 (After Schulze. From Lankester's Treatise on Zoology.) ex.c, Exhalant canals. sg.tr, Subgastral trabecular layer. d.m, Dermal membrane. g.m, Gastral membrane. sd.tr, Subdermaltrabecularlayer. G.C, Gastral cavity. fl.c, Flagellated chambers. loose trabecular tissue in which the canals are represented by irregular spaces. The Tetraxonida appear to have suffered reduction in the size of the flagellated chambers at a very early date, and it is of this group especially that the Rhagon type is characteristic (e.g. Plakina, fig. 4). The Euceratosa exhibit a beautiful series, Q0 0 10t 'h) O \- o a ) set in 0 or 00 OQ Q6 00 0 04 beginning with forms (Aplysillidae) having large sac-shaped chambers To this type (fig. 4, f) the name " eurypylous " has been given, and like those of Hexactinellids and ending with forms (Spongiidae, we may include in it cases where there is only a single prosopyle, and Euspongia, figs. 6, 8, 9) having small spherical chambers. Along all four lines of descent it is probable that folding of the choanosome, or chamber-bearing layer of the sponge-wall, has played a very important part in the evolution of the canal-system. This folding is very clearly seen in the Hexactinellida and in such forms as Oscarella (l\/Iyxospongida) and Plakina (Tetraxonida). By this process inhalant and exhalant canal-systems have been formed, and then the ends of the inhalant canals have in most cases been closed in by development of an ectoseme, as in Plakina trilopha and Stelletta phrissens (fig. 20). In the majority of cases (e.g. (After Sollas.) Euspongia) the folding has become so complex that it is no longer recognizable as such, and the origin of the now well-defined inhalant and exhalant canals is completely disguised. in many cases the principal exhalant canals may be surrounded by a layer of tissue of considerable thickness in which there are no flagellated chambers at all, known as the endosome, so that the folded choanosome may be sandwiched in between ectosome' on the outside and endosome on the inside. The manner in which the flagellated chambers communicate with their respective branches of the inhalant and exhalant canal- (After Sollas.) system varies considerably in different forms, and the following types are recognizable, though by no means sharply distinguished from one another. In the more primitive forms (e.g. Hexactinellida, Aplysillidae, Spongeliidae) each chamber is provided with several prosopyles and receives its water supply direct from relatively large inhalant canals or even lacunae, discharging it again through a wide mouth (apopyle) into a relatively large exhalant canal or lacuna which also receives water directly from other chambers. perhaps even a short, narrow inhalant canal. In more advanced forms the water is discharged from each chamber through a narrow exhalant canaliculus (aphodus) peculiar to itself, and thence into wider canals. This is known as the "aphodal " type (e.g. Cydonium, fig. 21). In the " diplodal " type there is a special inhalant canaliculus (prosodus) as well as a special aphodus to each chamber, with usually, at any rate, oniy a single prosopyle (e.g. Corticium, fig. 22). The progress from the eurypylous to the diplodal condition is accompanied by a corresponding increase in the development of the mesogloea, whereby the canals are greatly .Y restricted in diameter, and at the same time the mesogloea (After F. E. Schulze.) tends to lose its transparent FIG. 22.-Part of a section of gelatinous character and to Corticium candelabrum, O.S., show-become compact and granular. ing diplodal type of canal-system. With the growth of the ectc- The canal shown on the left is some we necessarily get a inhalant and that on the right (e) corresponding development of exhalant. the proximal portion of the inhalant canal-system. At first the ectosome is merely a thin membrane, the dermal membrane, pierced by the inhalant pores, which are usually arranged in groups. Beneath the groups of pores (pore-areas) lie spacious sub-dermal cavities which form the commencement of the inhalant canal-system in the choanosome. In more advanced types the ectosome becomes greatly thickened and may be specially strengthened in a variety of ways to form a cortex. The inhalant pores now no longer lead directly into the subdermal cavities, but first into a series of cavities lying in the cortex and known as chones, which may be separated from the underlying subdermal cavities (sub-cortical crypts) by definite sphincters (Cydonium, fig. 23). The arrangement of the oscula and pores on the surface of the sponge varies greatly in different types, and sometimes gives rise to very striking modifications of the external form. The oscula or vents are usually relatively large openings situated on the more prominent parts of the sponge, often on special elevations. Occasionally they are replaced by sieve-like oscular areas (e.g. Geodia perarmata), a modification which doubtless serves to prevent foreign bodies from entering the wide exhalant canals. The inhalant pores may be irregularly scattered over the surface of the sponge or collected in more or less well-defined pore-areas. In cup-shaped sponges the pores are usually confined to the outer and the oscula to the inner surface. In flabellate sponges we find pores on one side and oscula on the other. In Tedania actinii{ormis, a deep-sea form. the pores are restricted to a narrow band surrounding the columnar body of the sponge just beneath the flattened top, which bears the vents; thus they are kept from being choked up by the soft ooze on which the sponge lies. In Xenospongia, a flattened discoid form, they are confined to narrow grooves on the upper surface, the chief of which run round the margin of the disk. In Esperella murrayi the pores are also confined to special grooves on the surface of the sponge, and in both these cases the grooves can apparently be opened and closed by special bands of muscle-fibres, and the supply of water thus regulated. In some species of Latrunculia we find the surface of the sponge covered with (After Sullas.) conspicuous projections of two kinds, some conical and bearing each a single vent, others truncated at the top and bearing the inhalant pores. Skeleton.—The original ancestral form (Protolynthus) from which all the Porifera are supposed to be descended, probably possessed no proper skeleton at all, and this condition has been retained in the existing Myxospongida, although these sponges have made considerable progress in the evolution of their canal-system. There appears to be little doubt that the Myxospongida are primitively devoid of skeleton, and in this respect they must becarefullydistinguishedfrom thegenusChondrosia, in which the skeleton has been secondarily suppressed, as well as from numerous and divers species in which the proper skeleton has been more or less completely replaced by grains of sand or other foreign bodies. The Calcarea, Triaxonida, Tetraxonida and Euceratosa, except in cases of extreme degeneration, all possess a well-developed proper skeleton. As this skeleton has been independently evolved in each of these great groups it is necessary to deal with it separately in each case. Calcarea.—The skeleton in this group is composed of spicules of crystalline carbonate of lime (usually calcite), developed within special mother-cells or scleroblasts. Each spicule is enclosed in a delicate membranous spicule-sheath and contains an axial thread of organic matter. Three main types of calcareous spicule are met with, triradiate, quadriradiate and monaxon (fig. 24). The triradiates and quadriradiates, however, are not simple spicules, but spiculesystems formed of three or four rays each originating independently from its own scleroblast (actinoblast) and all uniting together secondarily. There is reason to believe that this may also sometimes be the case with the monaxon or oxeate spicules. In the most primitive triradiate spicules all three rays lie in thequadriradiate spicules. These may be sagittal, in which case the oral rays are turned towards the osculum while the basal ray is directed downwards. If there is an apical ray it projects into the gastral cavity. The walls of the radial chambers are supported by a special " tubar " skeleton (cf. fig. 14), consisting exclusively of triradiates with their basal rays directed towards the distal end of each chamber. The oral rays are spread out at right angles to the length of the chamber, and as several spicules generally lie at the same level the tubar skeleton forms a series of more or less definite joints and is said to be " articulate." This type of skeleton is almost invariably associated with the Syconoid type of canal-system. In the genus Sycon itself we find the distal ends of the chambers specially protected by tufts of monaxon spicules (fig. 14), but the next great advance in the evolution of the skeleton is brought about by the development of a dermal cortex, in which a special dermal skeleton is developed. This is well seen in the genus Ute (fig. i5). After this the skeleton of the chamber layer in the sponge-wall begins to undergo modifications, some of which are obviously correlated with the gradual change of the canal-system from the Syconoid to the Leuconoid condition (cf. figs. 16 and 17). Finally all trace of the articulate tubar skeleton is lost, and we get a " parenchymal " skeleton of scattered radiate spicules in the chamber layer. The skeleton of the chamber layer, no matter what the type of canal-system, may be supplefnented by large subdermal sagittal triradiates or subdermal quadriradiates (fig. 17), whose basal or apical rays project inwards from the dermal cortex (Heteropidae and Amphoriscidae). Very generally a special " oscular " skeleton is developed in the form of a fringe of long monaxon spicules around the vent. Various aberrant types of skeleton are met with in the group. In the genus Lelapia we find a partly fibrous skeleton, in wjtich the fibres are composed of bundles of triradiates shaped like tuning-forks (fig. 24, o), and in Petrostoma the main skeleton is formed of calcareous spicules actually fused together. In Astrosclera (fig. 25) a very anomalous type of calcareous skeleton is found, consisting of spherical masses of arragonite, each originating in a special scleroblast and having a radiate structure, recalling that of a siliceous (After W. J. Sollas.) FfG. 26.—Typical Siliceous Megascleres. a, Diactinal monaxon (oxeate). g, b, Style. c, Triact. d, Primitive tetraxon (calthrops). e, Hexact. f, Polyaxon desma. s (After E. A. Minchin. From Lankester's Treatise on Zoology.) same plane. Three chief varieties may be distinguished : (I) Regular (fig. 24, b), with all the rays and all the angles equal; (2) Sagittal (fig. 24, c, d, 1, &c.), with two of the rays or two of the angles forming a pair, differentiated in some respect from the remaining ray or angle, the paired rays being termed "oral" and the odd ray " basal "; (3) Irregular (fig. 24, p), when conforming to neither of the above types. It has been proposed to draw a very sharp distinction between " equi-angular " triradiates and " alate " forms (in which the angle between the oral rays differs from the paired angles), but it may be doubted whether such a distinction has any great value. The quadriradiate (fig. 24, e, f, k, m) is formed by the addition of an " apical " or "gastral " ray to the three " facial " rays of the triradiate; this ray lies in a plane at right angles to that of the facial rays. The monaxon spicules (fig. 24, it, i, q, r, s) are straight or curved and the two ends are usually more or less sharply differentiated from one another. In all these spicules the form and arrangement of the rays is usually clearly correlated with their position in the sponge in such a manner that they are specially adapted for the work which they have to do. The arrangement of the spicules in the case of the genus Leucosolenia has been dealt with above, and we must pass on at once to the Calcarea Heterocoela. In this group the skeleton exhibits an evolutionary series no less remarkable than that of the canal-system. We may take as a convenient starting-point the genus Sycetta, a typical Syconoid form, with the flagellated chambers radiating independently from the central gastral cavity. The wall of the gastral cavity is supported by a gastral skeleton of triradiate or (After J. J. A Lister.) Fie. 25.—Astrosclera willeyana (Lister). A, Entire sponge (x 3) : p.s., upper surface with openings of canal-system; b, base of attachment. B, Section of skeleton: sph, spherules of arragonite; c, canals. Sterraster (often regarded as a microsclere). h, Part of section of sterraster, showing two rays united by intervening silica. sterraster. These bodies become closely packed together over large areas, and give the sponge a stony hardness. Hexactinellida.—In this group the skeleton is composed of spicules of colloidal silica deposited in concentric lamellae around slender axes of an organic substance which in life occupies the " axial canal " of the spicule. Although varying greatly in detail and often exhibiting great complication or, it may be, reduction in structure, these spicules are all referable to the same fundamental triaxonid and hexactinellid type, characterized by the possession of three axes intersecting each other at right angles and each thereby divided into two rays or actines (fig. 26, e). According as one, two, three, four or five of these actines are suppressed we distinguish between pentact, tetract, triact, diact and monact spicules, and these may be further subdivided according to special modifications of the rays due to secondary branching, ornamentation by spines, find some of the spicules enlarged to form megascleres and others reduced to form microscleres. The megascleres play the principal part in building up the skeleton while the microscleres are usually scattered through the mesogloea. Triaene Series of Megascleres.—When three rays (cladi) of the tetract resemble one another, while the fourth (shaft) differs in some respect the spicule is termed a triaene. The simplest form is the plagiotriaene (fig. 29, 2), with three short simple cladi and an elon- V rl h~ (After F. E. Schulze.) a, Dagger. d, Amphidisc. f, Tetract (staurus). b., c, Pinuli. e, Pentact. g, Diact (rhabdus). knobs, &c., or curvature, or to excessive development of certain rays as compared with the remainder. Some of the most characteristic of these special types are represented in figs. 27 and 28. Two of them require special notice on account of their importance in the classification of the group. These are the hexaster and the (After F. E. Schulze.) Hexactinellida. a, Uncinaria; b, Clavula; c, Scopula. amphidisc. A hexaster (=rosette) is a perfectly symmetrical hexact whose actines branch out into secondary or terminal rays, in a star-like manner (fig. 30, t). Various sub-types are distinguished according to the character of the rays (floricome, plumicome, &c.). An amphidisc (fig. 27, d) is a diact spicule consisting of two opposite rays each of which terminates in a disk-like or spherical expansion surrounded by marginal teeth. In some cases the spicules all remain disconnected from one another (Lvssacine condition), in others some of them may be united by siliceous cement into a continuous framework (Dictyonine condition), and the distinction between these two types of arrangement was for a long time regarded as indicating a primary sub-division of the Hexactinellida into Lyssacina and Dictyonina, but this subdivision has now been abandoned. The term prostalia is applied to spicules which project freely from the surface of the sponge, and these are further distinguished as basalia, pleuralia and marginalia, according to their position at the base of the sponge, on the sides, or round the margin of the osculum. The basalia frequently form a root-tuft for attaching the sponge to the sub-stratum (Hyalonema, Euplectella) and commonly have anchor-like distal extremities. They may be extremely long, as in the well-known " glass-rope " of Hyalonema. In the remarkable genus Monorhaphis we find a single gigantic diact spicule, which may attain a length of two or three feet and the thickness of a lead pencil, transfixing the body of the sponge like a skewer from above down-wards. A special dermal skeleton is usually formed by a number of spicules distinguished as dermalia, and a gastral skeleton may be similarly formed by special gastralia surrounding the central gastral cavity. Between the dermal and gastral skeletons another set of spicules, known as parenchymalia, form the most important part of the skeleton, supporting the chamber-layer and adjacent tissues. The distinction into large megascleres and small microscleres is perhaps less well marked in this group than in the Tetraxonida. Tetraxonida.—Here, again, the spicules are composed of colloidal silica deposited around organic axial threads. The starting _point in the evolution of the very complex series of tetraxonid spicules is the primitive tetract or calthrops, characteristic of the most primitive members of the group (e.g. Plakina). This fundamental ground-form (fig. 26, d) consists of four rays or actines of equal length, which all meet one another at equal angles in the centre of the spicule, while their apices would occupy the four angles of a regular pyramid whose sides are four equilateral triangles. It is thus both tetraxonid (with four axes) and tetractinellid (with four rays). In Plakina the spicules are all of about the same size, neither very large nor very small, but in higher forms we usually Tetraxonida. 1, Primitive tetract. 14a, 14b, Pseudasters. 25, Chiaster. 2, Plagiotriaene. 15, Cladotylote. 26, Oxyaster. 3, Dchotriaene. 16, Acanthoxeate. 27, Aster with 4, Discotriaene. 16a, Pseudaster (am- branching rays. 5, Anatriaene. phidisc). 28, Rhaphis or tri- 6, Protriaene. 17, Strongyle. chite. 7, 8, Reduced tri- 18, Tylote. 29, Trichodragma. aenes, becoming 19, Cladostrongyle. 30, Sigmata. monaxon. 20, Rhabdocrepid 31, Isochela. 9, Tetracrepid desma. (monocrepid) 32, Anisochela. to, Primitive diact. desma. 33, Diancistron. 11, Oxeate. 21, Aster. 34, Toxon. 12, Style. 22, Spheraster. 35, Labis (forcipi- 13, Tylostyle. 23, Sterraster. form). 14, Acanthotylostyle. 24, Spiraster. gated shaft, the angles all remaining approximately equal. If the angles between the cladi and shaft become approximately right angles we have an orthotriaene. If the cladi point forward, we have a protriaene (fig. 29, 6). If the cladi are turned backwards towards the shaft we have an anatriaene (fig. 29, 5). If the cladi branch each into two we have a dichotriaene (fig. 29, 3)• If the cladi are expanded laterally and fused together to form a plate, while the shaft is reduced, we have a discotriaene (fig. 29, 4). The cladi may be reduced in size or even suppressed (fig. 29, 7, 8), leaving only the shaft, which may be either sharp at each end (oxeate) or sharp at the apex and rounded at the base (stylote). The spicule has now become monaxonid or monaxonellid (i.e. with a single axis) and monactinellid (with only a single ray) ; but this condition may also be arrived at in a different way, as we shall see directly. The tetracrepid desma (fig. 29, 9), characteristic of many Lithistids, has been derived from the primitive tetract by ramification of the ends of all the rays. Monaxonid Series of Megascleres.—We have already seen, in Plakina, how a diactinellid spicule may arise by suppression of two rays of the tetract (fig. 5). At first the two remaining axes a, c, d, e, f, g, h, .7, are distinctly indicated by the presence of an angle in the middle of the spicule (fig. 29, io) ; by straightening out of this angle we reach a monaxonid but diactinellid condition—the diactinellid oxeate, with the organic centre of the spicule in the middle (fig. 29, it). By rounding off of both ends this form passes into the strongylote (fig. 29, 17), then if both ends become enlarged into knobs it is said to be tylote (fig. 29, i8). If one end only is rounded off, which apparently usually takes place by suppression of one ray, while the other remains sharp, the spicule is termed stylote (fig. 29, 12). It is now monactinellid as well as monaxonid. If the blunt end of the style enlarges to form a knob we have the tylostyle (fig. 29, 13). Acanthoxeates (fig. 29, 16), acanthostyles and acanthotylostyles (fig. 29, 14) are formed by the development of spines on the surface of the spicule. The development of large recurved spines at the apex of a tylostyle gives us the cladotylote or grapnel spicule (fig. 29, 15), which simulates an anatriaene. By enlargement of the spiny base of an acanthotylostyle and suppression of the shaft we get forms which simulate astrose microscleres and may be called pseudasters (fig. 29, 14a, 14b). Pseudasters may also be developed by shortening up of acanthoxeates, accompanied by enlargement of the spines (e.g. Spongillinae, fig. 29, 16a). The exotyle appears to have been formed by enlargement of the outer end of a radially placed oxeate at the surface of the sponge. By ramification of both ends of a diactinal megasclere we get the monocrepid desma (fig. 29, 20), characteristic of certain Lithistids and closely simulating the tetracrepid desma. By ramification of one end of a strongylote spicule we may get a cladostrongyle (fig. 29, 19). Diactinal Series of Microscleres.—The starting-point of this series is the primitive angulate, diactinal oxeate (fig. 29, io). This has given rise to long hair-like forms or rhaphides (fig. 29, 28), short hair-like forms associated in bundles and called trichodragmata (fig. 29, 29), bow-shaped forms or toxa (fig. 29, 34), and C- and S-shaped forms or sigmata (fig. 29, 30). From the sigmata may be (After Sollas.) (sigmaspires). 1, Modified isochela of Melonanchora. m, Spheraster. n, o, p, Oxyasters. q, r, Reduced asters. s, Microxeate. t, Hexaster (rosette). derived the diancistra (fig. 29, 33), shaped like pocket-knives with a blade half open at each end, and the wonderful series of chelae (fig. 29, 31, 32), in which each end branches into a number of sharply recurved teeth. These chelae are characteristic of the family Desmacidonidae, and exhibit great variations in detail, while each particular form is remarkably constant in the species in which it occurs. The most curious and aberrant are those of Melonanchora (fig. 30, 1) and Guitarra. In isochelae the two ends of the spicule are equal, in anisochelae they are unequal. Astrose or Polyaclinal Series of Microscleres.—For the beginning of this series we must go back to the primitive tetract. Reduction in size, sometimes accompanied by increase in the number of rays, has given rise to the oxyaster (fig. 29, 26), with sharp rays and no conspicuous centrum. The development of a distinct centrum from which numerous rays come off gives us the spheraster (fig. 29, 22). In the sterraster (fig. 26, g, h), characteristic of the family Geodiidae, numerous slender rays become fused together side by side to form a solid ball. In the spiraster (fig. 29, 24) the centrum appears to have become elongated and twisted into a spiral. The rays of the aster may terminate in knobs as in the chiaster (fig. 29, 25), or they may become branched (fig. 29, 27). Arrangement of the Skeleton in the Tetraxonida,—The most primitive type of skeleton arrangement in this group was probably very similar to that which we still find in Plakina or Dercitopsis, butwithout any special dermal spicules, the skeleton consisting exclusively of small isolated tetracts irregularly scattered through the mesogloea between the chambers. We may call this the scattered or diffuse type of skeleton. With the development of an ectosome —whether thin dermal membrane or thick cortex—a special dermal skeleton arose. Sometimes this consists of small specially differentiated dermal spicules—candelabra in Plakina, oxeates in Dercitopsis—but a much more important series of modifications was (After W. J. Sollas. ) initiated by the development of the triaenes. The cladi of these spicules are commonly extended in or beneath the ectosome and form a very efficient dermal skeleton, while the shafts are directed centripetally through the choanosome. In the genus Discodermia the discotriaenes form a continuous dermal armour of siliceous plates. When anatriaenes and protriaenes are developed their cladi commonly project beyond the surface of the sponge and render it more or less strongly hispid, thus forming a protection from the attacks of enemies. The shafts of the triaenes, though greatly reduced in Discodermia, usually become very much hypertrophied and may be grouped together in bundles, often associated with oxeate spicules. These spicules, or bundles of spicules, now form the principal part of the skeleton, and inasmuch as they radiate from the interior towards the surface of the sponge we distinguish this as the radiate type of skeleton. The skeleton of the vast majority of Tetraxonida is either actually radiate in structure or derived from the radiate type by further modification. In many Stellettidae, for example (fig. 31), we have a typical radiate skeleton in which a large number of the spicules retain the primitive tetractinellid form, though associated with oxeates, while in Tethya the skeleton is arranged in a similar manner but only monaxonid spicules are present. From the radiate we pass to the reticulate type of A `L (After Minchin and Deady. A, B, C from Lankester's Treatise on Zoology, D from Trans. o,^ Zool Soc. of London, vol. xii.) sp, Spicules; spg., Spongin; m.f., Primary fibres; cf., Secondary (connecting) fibres. .0 b, Sigmata Toxon. Spiraster. Sanidaster. Amphiaster. Sigma. k, Isochelae. End of a chela, showing the teeth. skeleton which characterizes the majority of the so-called Monaxonellida. This is derived from the former by the establishment of secondary spicule-bundles connecting the primary or radial bundles together, and the transit%on is usually accompanied by loss of the cladi of the triaenes and by the development of a massive irregular form on the part of the entire sponge. An intermediate condition is found in some of the massive species of Tetilla (e.g. T. limicola), in which the spicule-bundles are very well defined and form distinct primary " fibres " in the interior of the sponge, but no distinct secondary or connecting fibres are yet developed. (After Lendenfeid. Modified from Lendenfeld's Horny Sponges, by permission of the Royal Society of London.) In the Sigmatomonaxonellida, derived from the Tetillidae, the reticulate type of skeleton is almost universal, and in this group an entirely new element is introduced into the skeleton with the development of a " horny " cementing material (spongin) which unites the spicules together in the fibres. At first small in quantity (Reniera, fig. 32, A), the spongin cement gradually increases in proportion to the spicules until in many Chalininae (fig. 32, B, C) and Desmacidenidae the spicules become completely embedded in it, and the fibres may be formed chiefly of spongin, with only a core of spicules. The complete enclosure of the spicules by spongin at a very early stage cuts off their food supply and causes arrest of development. Finally, in some Chalininae (fig. 32, D) and Desmacidonidae the spicules entirely disappear from the interior of the fibre, and if at the same time they happen to be absent from the intervening mesogloea we get a skeleton composed exclusively of horny matter or spongin, to which the term pseudoceratose may be applied. In the sub-family Ectyoninae the skeleton be-comes modified in an interesting manner by the development of " echinating " spicules, usually acanthostyles or acanthotylostyles, whose bases are cemented on to the fibre by spongin while their apices project into the surrounding soft tissues. These doubtless serve as a defence against internal parasites. In Agelas these echinating spicules may persist after the spicules have entirely disappeared from the interior of the strongly developed horny fibre. In the Axinellidae all the spicules in the fibres are typically more or less echinating in character and the fibres become plume-like. Very frequently a special dermal skeleton is developed in the ectosome altogether distinct from that formed by the cladi of the triaenes (when these are present). Thus in the Geodiidae (ng. 23) the thick cortex is almost filled with densely packed sterrasters. In many forms there is a dense layer of small radially arranged monaxons at the surface of the sponge, whose projecting a, has: form an efficient protection. In the reticulate forms the ectosome is usually a thin dermal membrane supported by a reticulate dermal skeleton of slightly different structure from the " main " skeleton. In cases where a special stalk or a root-tuft is developed we also find a special and appropriate skeleton in connexion there-with. In the so-called Lithistida alone amongst the Tetraxonida do we find the spicules (desmas) united together by silica to form a coherent skeleton, sometimes of stony hardness, very different from the elastic, flexible skeleton resulting from the development of spongin, and analogous to the condition met with in the Dictyonine Hexactinellids. The microscleres usually play quite a subordinate part in the formation of the skeleton, being scattered irregularly throughout the mesogloea, though sometimes (Geodia, Tethya) the asters may form a definite cortical layer. Euceratosa.—In the true horny sponges, if we neglect for the moment the presence of foreign bodies, we may say that the skeleton consists from the first exclusively of spongin, secreted (by specialspongoblasts) in concentric layers to form very well defined fibres. In the most primitive forms (Aplysillidae) this horny skeleton is dendritic in arrangement (fig 33), composed of fibres which rise vertically upwards from the base of the sponge (where they may be expanded to form a horny basal cuticle which serves for attachment) and ramify towards the surface, where their apices push against the dermal membrane and cause it to project in the form of " conuli." No reticulation is formed in the simplest cases (A plysilla, Dendrilla), but in Megalopastas secondary connecting fibres are established (in relation, doubtless, to the increase in size and massive form of the sponge), and the skeleton thus simulates the pseudoceratose reticulate type of the Sigmatomonaxonellida. In Darwinella we have, in addition to the dendritic skeleton, isolated " spicules " of spongin scattered irregularly through the mesogloea. The presence of these spicules, which are sometimes, though by no means always, hexactinellid in form, has given rise to much speculation as to the possible relationship of the Aplysillidae to the siliceous Hexactinellida. Until we know more about their origin, however, we may perhaps best regard them simply as detached portions of the general skeleton secreted by isolated groups of spongoblasts. The genus Megalopastas forms a natural transition to the Spongeliidae, in which the reticulation of the horny skeleton is an almost constant feature, and in which the tendency to supplement or replace the spongin by foreign bodies (sand. broken spicules) is very strongly marked. In extreme cases the skeleton is composed almost exclusively of sand (e.g. Psammopemma), and the whole sponge looks like a mass of sand stuck together by a minimum of soft tissues and spongin cement. Such " arenaceous " sponges also occur in other groups (e.g. Desmacidonidae). The culminating point in the development of the true horny skeleton is found in the Spongiidae (e.g. Euspongia), but even in the bath sponge (fig. 6) we commonly find sand grains or other foreign matter in the interior of the primary fibres. The value of the sponge for domestic purposes depends upon the softness and elasticity of the fibre, the closeness of the meshes, and the relative absence of sand. histology. There are two primary tissue-forms in sponges, the flat pavement epithelium and the epithelium composed of choanocytes or collared cells. The former covers the whole of the external surface of the sponge and, except in the simpler Calcarea Homocoela, it also lines a considerable portion of the canal-system. The latter lines practically the whole of the primitive gastral cavity in the Calcarea Homocoela, but in all higher types becomes restricted to well- defined " flagellated chambers." A gelatinous " mesogloea;" which must be regarded primarily as an intercellular substance, appears between the primitive outer and inner layers of the sponge-wall. This contains primitive amoeboid wandering cells (archaeocytes), 2 (After Dendy. From Quart fourn. Micro. Science, new series, vol. xxxv., by permission of J. and A. Churchill.) I, Pavement epithelium from the upper surface of an oscular diaphragm of Vormaeropsis wilsoni. 2, Chamber diaphragm of Vermaeropsis macera; mus.c, Myocytes; ex.op, Exhalant aperture of flagellated chamber. 4, 5, Amoebocytes of Leucandra phillipensis (the one shown in 5 appears to be feeding by means of pseudopodia upon the collared cells (c.c.) of a flagellated chamber). 6, Section across an inhalant canal (i.c.) of Ute syconoides, showing an ovum (ov.) suspended from the wall, apparently awaiting fertilization; sp, spicules. 3 which give rise to the ova and spermatozoa, and also various other cells which are now generally believed to migrate into it from the primitive pavement epithelium (dermal epithelium) of the outer surface, such as scleroblasts, various connective tissue elements and contractile fibres. Pavement Epithelium (fig. 34,I).—This always consists of a single layer of polygonal cells, which are usually flat and very rarely (Oscarella) provided with cilia or flagella. They may be glandular and may secrete a definite cuticle (as in many Euceratosa). They may also be highly contractile. Porocytes.—In certain Calcareous sponges (Leucosolenia) it has been shown (by E. A. Minchin) that the primitive inhalant pores (prosopyles) are formed as perforations in certain of the pavement epithelium cells, which acquire a tubular form and extend through the mesogloea from the dermal to the gastral surface. The outer portion of each porocyte forms a contractile diaphragm which doubtless regulates the admission of water to the gastral cavity. The porocytes are sometimes conspicuous on account of their highly granular character. Scleroblasts.—We may distinguish three kinds of scleroblasts, according to the chemical character of the skeletal material which they secrete; these are calcoblasts, silicoblasts and spongoblasts. The calcoblasts and silicoblasts (fig. 35, h–n) form their respective spicules, at any rate in the first instance, as intra-cellular (perhaps sometimes intra-syncytial) secretions, though we must suppose (After Schulze and Sollas.) a, Collencytes from Thenea muricata. b, Chondrenchyme (with spicules) from Corticium candelabrum. c, Cystenchyme, from Pachymatisma johnstoni. d, Desmacyte, from Deegmastra normani. e, Myocytes and collencytes, from Cinachyra barbata. f, Thesocyte, from Thenea muricata. g, Collared cell (choanocyte), from Sycon raphanus. hen, Silicoblasts or mother-cells, in which different forms of siliceous spicules are being secreted. that in the case of large spicules the later stages in growth are accoms plished by the activity of several or many scleroblasts in co-operation. The spongoblasts (fig. 7) appear to co-operate with one another in the formation of the spongin fibre from the beginning. They are found only around the young, growing fibres, where they occur in large numbers, forming a kind of sheath of somewhat flask-shaped cells, each placed at right angles to the surface of the fibre and with the nucleus in its broad distal end. The spongin is secreted in concentric lamellae and is obviously intercellular in origin, and probably of the same nature as the cuticle which often occurs on the surface of the sponge. Connective-tissue Elements.—The following are the chief forms assumed by the mesogloea according to the nature of its connective-tissue cells and intercellular substance. (a) Collenchyme, consisting of a clear gelatinous matrix with branching stellate collencytes (fig. 35, a) embedded in it; (b) Sarcenchyme, in which the quantity of intercellular matrix is greatly reduced and the connective-tissue cells are closely packed together; (c) Cystenchyme (fig. 7, Coll , fig. 35, c), consisting of close-packed, oval, vesicular cells with fluid contents and strands of protoplasm radiating from the nucleus to the periphery; (d) Chondrenchyme (fig. 35, b), somewhat resembling cartilage in texture and with a very large amount of intercellular matrix. The name desmacytes has been given to certain slender connective-tissue fibres (fig. 35, d) often united in dense bundles or layers, which occur especially in the ectosome of many Tetraxonida, giving rise to a fibrous cortex of leathery consistence. Contractile Fibres.—Muscular fibres or myocytes (fig. 35, e) are of common occurrence, especially in relation to various parts of the canal-system, the diameter of which appears to be regulated by their agency. They may form definite sphincters around the vents or in other places (fig. 34, 2), or they may form transverse bands lying in the floor of pore-bearing grooves, by the contraction of which the lips of the groove are doubtless approximated and the in-current stream of water shut off (Esperella murrayi, Xenospongia patelliformis). Endothelial Cells.—In many sponges the developing embryos are enclosed in definite capsules composed of flattened polygonal cells, the whole being embedded in the mesogloea. The origin of the endothelial cells forming the capsules is doubtful. They sometimes aid in the nutrition of the developing embryo (e.g. in Stelospongus flabelliformis). No nervous elements, nor sensory cells of any kind, have as yet been recognized with any degree of certainty in sponges, in spite of various heroic attempts to demonstrate their existence. Collared Cells or Choanocytes (fig. 35, g).—These are quite the most characteristic histological elements met with in sponges. Although exhibiting various minor differences in structure, and still more as regards size, they always show the same essential features. Each consists usually of an oval or rounded body (frequently appearing polygonal from the pressure of its fellows) surmounted by a more or less cylindrical or funnel-shaped collar, which surrounds a single long, whip-like flagellum projecting from the apex of the cell. The collar is a filmy, transparent ex-tension of the cytoplasm (cell - protoplasm), which can be completely with-drawn. The flagellum may also be withdrawn, and in preserved specimens neither collar nor flagellum is usually visible. The cell is usually broadest at the base and narrowed to form a neck or " collum," beneath the collar. The nucleus may be situated either at the base or at yl.,. the apex of the cell-body or between the two. The collar itself is often a more complicated structure than appears at first sight. It may be provided with one or two transverse hoops, presumably serving to stiffen it (Ascandra falcata). In many cases the collars of adjacent choanocytes have been observed to be connected by a definite membrane which stretches from one to the other at the level of their margins. This is known as Sollas's membrane, but it is apparently not a permanent structure, and the circumstances under which it appears require elucidation. In the Hexactinellida the form of the collared cells appears to be somewhat unusual (fig. 36). Archaeocytes.—The term " archaeocytes " has been applied to certain undifferentiated amoeboid cells which make their appearance at an extremely early stage in the ontogeny, and some of which persist throughout life, with little, if any, modification, as the amoebocytes of the adult sponge, while others become germ-cells, differ entiated into ova and spermatozoa. (After F. E. Schulze.) Amoebocyles.—These are amoeboid cells closely resembling the leucocytes or white blood corpuscles of higher animals. They commonly have blunt, lobose pseudopodia and the cytoplasm is generally more or less densely charged with refractive granules. They have the power of wandering from place to place through the mesogloea (fig. 34, 3–5)• (After Polejaeff and Schulze.) a–h, Development of Spermatozoa in Sycon raphanus; h, Mature Spermatozoa; j, Sperm-ball in Mesogloea of Oscarella lobularis; k, Mature Spermatozoon. Germ-Cells.—The ova (fig. 34, 6) are formed from amoehocytes, which grow to a large size and finally withdraw their pseudopodia and acquire a rounded form. They have large nuclei with a very distinct nuclear membrane and commonly a conspicuous nucleolus. The spermatozoa (fig. 37) closely resemble those of higher animals, consisting each of a small " head," composed chiefly of chromatin material, and a slender vibratile " tail " composed of cytoplasm. In this case the amoebocyte gives rise to a single sperm mother-cell (spermatocyte) sometimes enclosed in one or two covering cells. The nucleus of the spermatocyte undergoes repeated mitosis and a " sperm-ball " is produced which is either enclosed in the covering cell or in a special endothelium similar to that which surrounds the segmenting ovum. The germ-cells occur scattered through the mesogloea and are not aggregated in gonads, so that we cannot speak of " ovaries " and " testes " as in higher types: Reproduction. Reproduction in sponges may be effected in one of three ways: (I) The first is by vegetative budding, followed by separation of the buds and thus differing from the ordinary budding which leads merely to increase in the size of the sponge-colony. This process has been observed in many cases (e.g. Leucosolenia, Oscarella, Lophocalyx, Aplysilla). (2) The second way is by the formation of specialized reproductive bodies known as gemmules. This process is best known in the fresh-water sponges (Spongillinae), where it has been developed as a special means of tiding over unfavourable periods during which the parent sponge is liable to be destroyed by cold or drought. Each gemmule consists of an aggregation of amoeboid cells (statocytes) densely charged with nutrient granules and enclosed in a protective horny envelope which may he strengthened by a layer of special spicules. The ripe gemmule is very resistant to adverse conditions and is capable of remaining dormant for a lengthened period, and of developing into a new sponge on the return of favourable conditions. In temperate climates the gemmules remain dormant through-out the winter and develop in the spring, the development being very similar to that of an ordinary fertilized ovum except that it begins at the " morula " stage, with the numerous statocytes representing the blastomeres. (3) The third way is by the union of ova and spermatozoa to form zygotes, which undergo segmentation and develop into the adult through a more or less complex series of ontogenetic stages. Previous to fertilization the ovum undergoes a process of maturation accompanied by the extrusion of two polar bodies, as in higher animals. Very little is known about the actual process of fertilization, but it appears probable that this is effected in the inhalant canals of the parent sponge, where the ova have been observed suspended from the epithelial lining of the canal (e.g. in Ute, fig. 34, 6). After fertilization they appear, usually at any rate, to migrate back into the mesogloea, where they become surrounded by endothelial capsules and undergo segmentation. In Stelospongus flabelliformis the cells of the capsule are of gigantic size and are attached to the superficial blastomeres of the developing embryo by protoplasmic processes, through which, no doubt, nutriment is passed from the parent to the embryo. Embryology. The segmentation of the ovum appears to be in all cases complete or holoblastic, and the young sponge usually leaves the parent inthe form of a free-swimming ciliated larva, which, after fixing itself to some object, undergoes a metamorphosis and then grows into the adult form. The details of development appear to differ widely in different species and various interpretations have been placed upon somewhat limited and discrepant observations. One of the best-known cases is that of the calcareous genus Sycon (fig. 38). The fertilized ova develop into ciliated larvae within the parent sponge, embedded in the walls of the radial chambers, in their endothelial capsules. Each divides first into two, then into four, and then into eight equal and similar blastomeres by successive vertical clefts. The eight-celled stage (fig. 38, b, c,) has the form of a somewhat flattened cushion, with an axial cavity which is the beginning of the blastocoel or segmentation cavity. A horizontal cleft now divides each blastomere into a somewhat smaller upper and a somewhat larger lower portion, and the sixteen blastomeres arrange themselves in the form of a hollow sphere surrounding the blastocoel. The smaller cells multiply rapidly and become columnar, while still remaining as a single layer. Each one presently acquires a flagellum (" cilium ") at its outer end. The larger cells multiply more slowly and are characterized by their coarsely granular appearance. They are destined to give rise to the dermal layer and its derivatives (including archaeocytes ?) and never become flagellated.' The blastosphere or blastula (fig. 38, d, e,) is now complete, the blastocoel being completely surrounded by a single layer of cells differentiated, however, into two groups, gastral and dermal. The large granula (dermal) cells now become invaginated, but this f, Invagination of flagellated cells. g, Gastrula attached by oral face. h, Young sponge (Olynthus stage). j, Top view of young sponge. is only a temporary condition, probably to be explained as the mechanical result of the pressure of the spicules of the parent sponge. The so-called " pseudogastrula " thus formed escapes by rupture ' According to E. A. Minchin, the first-formed granular cells are " archaeocytes," which migrate into the interior of the larva while their place is taken by granular cells formed by modification of the neighbouring flagellated cells. The later-formed granular cells are destined to give rise to the dermal layer of the adult, while the remaining flagellated cells form the gastral layer. (After F. E. Schulze.) a, Ovum. b, c, Embryo with 8 blastomeres (b, top view, c, side view). d, Blastosphere (blastula). e, Larva at time of escape from parent. of the parent tissues into a radial flagellated chamber and passes to the exterior with the outgoing stream of water. The invaginated dermal cells are pushed out again and the " amphiblastula " swims away (fig. 38, e). (Possibly the granular dermal cells, by proliferation, may form a solid mass blocking up the blastocoel completely, so that we have a solid embryo.) The larva now fixes itself by the anterior flagellated pole (which, according to Schulze, becomes permanently invaginated, thus giving rise to a true gastrula, fig. 38, f, g), and the dermal cells spread themselves out over the gastral cells, which they completely cover. The fixed larva (" pupa ") consists of a solid mass of gastral cells enclosed in a single layer of now flattened dermal cells. Presently the gastral cavity appears (or reappears) in the middle, around which the gastral cells arrange themselves in a single layer. The young sponge elongates upwards, some of the dermal cells form porocytes which become perforated by prosopyles, others migrate into the gelatinous mesogloea and form scleroblasts, from which spicules are developed. The cells of the gastral layer acquire collars in addition to their flagella, an osculum is formed by perforation at the apex, and the young sponge begins to feed. It is now in the Olynthus condition (fig. 38, h) and is exactly comparable to a simple Leucosolenia individual. As it grows older radial flagellated chambers are budded out around the central gastral cavity and the collared cells lining the latter are replaced by pavement-epithelium derived from the dermal layer. An interesting account of the development of Leucosolenia (Clathrina) blanca has been given by E. A. Minchin. Segmentation is regular and complete, resulting in the formation of a hollow, ciliated, oval blastula (fig. 39, A), with a large blastocoel and a wall composed of a single layer of columnar flagellated cells and a pair of very large granular cells at the posterior pole. The latter are primitive archaeocytes and are destined to give rise to the amoebocytes and germ-cells of the adult. The flagellated cells will give rise to all the other cells of the adult, both dermal and gastral. The larva becomes free-swimming in this condition. Here and there individual flagellated cells (destined to form the cells of the dermal layer) lose their flagella and, becoming amoeboid, migrate into the blastocoel, which presently becomes completely filled with such cells. The larva is thus converted into a solid " parenchymula," in which the archaeocytes remain unchanged in their original position at the posterior extremity. It now fixes itself and flattens out upon the substratum in the pupal condition. During the metamorphosis Which now ensues the majority of the cells of the inner mass (dermal cells) pass out to the exterior again between the flagellated cells(gastral cells), over which they spread themselves in the form of a dermal layer of flattened epithelium. Some of the dermal cells, however, remain in the inner mass as porocytes; the primitive archaeocytes have divided up into amoebocytes; and porocytes, amoebocytes and the cells of the gastral layer are all crowded together in the interior of the pupa. The pupa now elongates vertically. A gastral cavity appears in the interior. The cells of the gastral layer arrange themselves around this cavity and develop their collars and flagella. At first, however, the gastral cavity is lined by the porocytes, which presently separate and migrate out-wards.' Scleroblasts migrate inwards from the dermal layer and secrete spicules. An osculum and prosopyles are formed as in Sycon and the Olynthus stage is reached. The development of sponges in general appears to be characterized by a remarkable want of uniformity in the arrangement of the different kinds of cells of which the larva is composed. Two, or possibly three, primary groups of cells are universally present; the flagellated cells, which will give rise to the collared cells of the adult, the non-flagellated (granular) cells, which will give rise to the dermal layer and its derivatives, and possibly the primitive archaeocytes (perhaps to be regarded as undifferentiated blastomeres). It may be considered as doubtful, however, whether the primitive archaeocytes can in all cases be distinguished from the primitive dermal cells. The latter are in some cases (amphiblastula type) grouped at the posterior pole of the larva (Sycon), while in other cases (pa.renchymula type) they may pass inwards and completely fill the interior, blocking up the blastocoel and perhaps also freely projecting at the hinder end (fig. 39, F). At the time of the metamorphosis the dermal cells pass to the outside and come to completely enclose the gastral cells, so that the two layers acquire their proper relative positions. The sponge larva in many respects closely resembles the Coelenterate " planula," with its ectoderm and endoderm, but it is very doubtful how far this comparison is valid, and in the present state of our knowledge it is perhaps better to avoid the use of the terms ectoderm and endoderm in dealing with the sponges altogether. The idea naturally suggests itself that the two primary layers of the Sponge correspond to those of the Coelenterate, but in a reversed position, the inner layer of the one being the outer layer of the other, and vice versa, and this idea has found expression in the name Enantiozoa which has been proposed for the group by Yves Delage, but which has not met with general acceptance. Physiology. Comparatively little is known of the physiology of sponges. The most obvious expression of the vital activity of the organism is the stream of water which flows in through the dermal pores or ostia and out through the vents or oscula. That this stream is maintained by the undulatory movements of the flagella of the collared cells there can be no doubt, but the fact that the movements of the flagella of different cells are not co-ordinated, so that they do not act in unison, indicates that the mechanical problem involved is not so simple as is usually supposed. There can he no doubt that the incoming stream brings with it minute food-particles, consisting of fragments of organic matter, alive or dead, and also the oxygen required for purposes of respiration; while the outgoing stream removes faecal products and waste matter (excreta). The rate of flow appears to be regulated by the opening and closing of the pores and vents, or of intermediate apertures such as the apopyles or exhalent openings of the flagellate chambers. This opening and closing may be effected by the activity of definite muscular sphincters (fig. 34, 2) or, in the case of some prosopyles, by the contractility of the porocytes themselves. The ingestion of the food particles is no doubt effected in large measure by the collared cells, which seem to feed much in the same manner as independent collared monads (Choanoflagellata). It seems not improbable that Sollas's membrane may be a temporary structure which assists in arresting food particles as they pass through the flagellate chambers. There is reason to believe also that amoebocytes (in this case therefore phagocytes) may capture minute organisms on their way through the canal system, and even porocytes are sometimes credited with this power. Digestion, no doubt, is, at any rate chiefly, intracellular. The amoebocytes probably serve not only to ingest food themselves but also to receive surplus food from the collared cells and distribute it through the sponge (fig. 34, 5)• Nothing definite is known as to the function of excretion, but here, as in the case of nutrition, it seems likely that collared cells and amoebocytes are both concerned. i The position of the porocytes inside the collared cells appears at first sight very anomalous, but Minchin has shown that this condition is actually repeated in the adult sponge every time the gastral cavity is obliterated by contraction. 1 (After E. A. Minchin.) A, Larva of Leucosolenia (Clathrina) blanca. B, Of Leucosolenia (Clathrina) reticulum. C, Young larva of Leucosolenia (or pseudogastrula stage of Sycon). D, Late larva of Leucosolenia (or newly hatched larva of Sycon). E, Larva of Oscarella. F, Parenchymula larva of a siliceous Mona xonellid (Myxilla). Sponges, as we have already seen possess no special nervous system and no special sense organs, and the power of response to stimuli appears to be very limited. Many sponges probably have the power of contracting as a whole, which may in some cases be due, in part at any rate, to the presence of bands of muscular fibres, and Sollas observes that in Pachymatisma irritation of the oscular margin is invariably followed after a short interval by a slow closure of the sphincter. The power of movement in adult sponges is, however, chiefly confined to individual cells acting independently. The young larvae, on the other hand, swim vigorously about by means of their cilia or flagella, whose movements must obviously be co-ordinated in order to ensure the progress of the entire organism in definite directions. The rate of growth of sponges appears to be very rapid. A British species of Hymeniacidon is said to form a crust measuring a foot in diameter in so short a period as five months. With this rapidity of growth must be associated the fact that many sponges, marine as well as fresh-water, appear to be annual. Distribution. The vast majority of sponges are marine, only a single sub-family, the Spongillinae, having acquired the habit of living in fresh water. The Spongillinae are, however, very widely distributed, being found in lakes and rivers in all parts of the world. Marine sponges occur everywhere, from low-water mark to the greatest depths, but certain localities. such as the Gulf of Manaar, Port Phillip and Port Jackson, appear to be much richer than others both in individuals and species. The Hexactinellida are essentially a deep-water group and are there-fore much more rarely met with than other forms. The Tetraxonida and Euceratosa abound in shallow and in moderately deep water, and a comparatively small number of species of Tetraxonida occur at great depths. Both are dominant groups at the present day, represented by very large numbers of species and individuals. The Myxospongida are comparatively rare and represented by very few species. The Calcarea are common. in the littoral region, especially in sheltered situations amongst rocks and seaweed. Most families and even genera of sponges enjoy a very wide geographical range, very many being cosmopolitan. Species are usually much more restricted in distribution, but even here there are some noteworthy exceptions, and future researches will probably show that many species from different localities which are at present regarded as distinct are connected by intermediate forms living in intermediate situations. There appears to be a well-marked relation between temperature and the power of spongin-secretion, and as a result we find that sponges with a really well-developed horny skeleton (whether Euceratosa or Pseudoceratosa) are usually only met with in comparatively warm waters. This fact brings about a striking contrast between the sponge-faunas of different latitudes. Classification. The classification of the Phylum Porifera, the characters of which have already been given, is as follows: Sub-phylum and Class Calcarea.—Sponges with a skeleton composed of carbonate of lime, commonly in the form of isolated spicules whose most usual shape is triradiate. Order z. Honiocoela.—Calcarea in which the gastral cavity and its outgrowths are lined throughout by collared cells. This order is sometimes divided into two families, Clathrinidae and Leucosoleniidae, but it is doubtful if this distinction can be maintained, and by some writers only a single genus .(Leucosolenia) is recognized. Order 2. Heterocoela.—Calcarea in which the original lining of the gastral cavity is partly replaced by pavement epithelium, so that the collared cells are confined to separate flagellated chambers. This order includes the living families Leucascidae, Sycettidae, Grantidae, Heteropidae, Amphoriscidae and Pharetronidae (with only two living representatives but numerous fossil forms). The relationships of the anomalous Astrosclera (fig. 25), for which the family Astroscleridae has been proposed by J. J. Lister, must still be regarded as problematical. Sub-phylum Non-Calcarea.—Sponges without any calcareous skeleton. Class and Order MYxosroNGIDA.—Sponges with no skeleton; with simple canal system and usually large flagellate chambers. (The absence of skeleton is primitive and not due to degeneration.) This class is sometimes divided into two families—Halisarcidae, with elongated, sac-shaped chambers, and Oscarellidae, with more or less spherical chambers. Class TRIAXONIDA (=HEXACTINELLIDA).—Sponges with a skeleton composed of siliceous spicules, either isolated or cemented together by silica, and either triaxonid and hexactinellid in form or derivable from the triaxonid and hexactinellid type. The canal system is simple and the flagellated chambers are large and sac-shaped, and more or less radially arranged in a network of trabecular tissue. Spongin is never formed. Order z. Amphidiscophora.—Triaxonida with characteristic amphidisc spictles, but no hexasters, and with a root-tuft of anchoring spicules. The family Hyalonematidae, including the well-known glass-rope sponges of the genus Hyalonema, is the only family recognized in this order. Order 2. Hexasterophora.—Triaxonida whose most characteristic spicules are hexasters. To this order belong the living families Euplectellidae, Asconematidae, Rossellidae, Euretidae, Melittionidae, Coscinoporidae, Tretodictyidae and Maeandrospongidae, and a number of extinct families such as the Ventriculitidae so commonly met with in the Jurassic and Cretaceous rocks. Class TETRAXONIDA.—Sponges with a skeleton composed of siliceous spicules, either isolated or cemented together (by silica or by spongin), and either tetraxonid and tetractinellid in form or derivable from the tetraxonid and tetractinellid type. The canal system is usually complex, with small, more or less spherical flagellated chambers. Grade TETRACTINELLIDA.—Tetraxonida in which some, at any rate, of the megasclergs retain the primitive tetractinellid form. No desmas are developed. Order z. Homosclerophora.--Tetractinellida in which microscleres and megascleres are not yet sharply differentiated from one another and no triaenes are developed. The canal system is comparatively simple. This order includes the family Plakinidae (see Plakina, ante) which forms the starting-point of the evolution of the class. Order 2. Astrophora.—Tetractinellida with triaenes and with astrose microscleres, without sigmata. This order includes the families Pachastrellidae, Theneidae, Stellettidae, Geodiidae. Order 3. Sigrnatophora.—Tetractinellida with triaenes, with sigmata for microscleres (when present), without asters. This order includes the families Tetillidae and Samidae. Grade (? order) LrrulsrIDA.—Tetraxonida in which the megascleres form desmas, typically united with each other by siliceous cement to form a continuous skeleton, often of stony hardness. This group includes both tetractinellid and monaxonellid forms and may possibly be of polyphy'.etic origin. The Lithistida bear the same relation to the other Tetraxonida that the dictyonine Hexactinellids bear to the lyssacine forms, but in the present state of bur knowledge it is hardly possible to trace the natural affinities of the numerous members of the group, many of which are only known in the fossil state. The following are the principal families: Tetraladidae, Desmanthidae, Corallistidae, Pleromidae, Neopeltidae, Scleritodermidae, Cladopeltidae, Azoricidae, Anomocladidae. Grade MoNAxoNEgLIDA.—Tetraxonida in which the primitive tetraxonid and tetracfinellid condition of the megascleres has been entirely lost through suppression of some of the spicule rays, so that none but monaxonellid megascleres remain. No desmas are developed. Owing to the extreme reduction or modification of the skeleton, leading in many cases to convergence, the classification of this group is extraordinarily difficult and the group is obviously not monophyletic. Order z. Astromonaxonellida.—Monaxonellida in which the microsclere, when present, is some form of aster. The members of this order are to be regarded as descended from aster-bearing tetractinellid ancestors. Families.—Epipolasidae, Tethyidae, Spirastrellidae (including Placospongiidae), Clionidae (the boring sponges), Suberitidae, Chondrosiidae. (In Chondrosia the skeleton is entirely suppressed, so that it simulates the Myxospongida.) Order 2. Sigmatomonaxonellida --Monaxonellida in which tht typical microscleres are sigmata, or other diactinal forms. Normal astrose microscleres are absent (though secondary pseudasters are occasionally present). The members of this order are to be regarded as descended from sigma-bearing tetractinellid ancestors. Families.—Haploscleridae (chief sub-families: Gelliinae, Renierina Chalininae, Spongillinae), Desmacidonidae (chief sub-families: Esperellinae, Ectyoninae), Axinellidae. Class and Order EUCERATOSA.—Non-calcareous sponges without siliceous spicules, but with a skeleton composed of horny fibres developed independently, i.e. not in relation to any pre-existing spicular skeleton. The skeleton is often supplemented, or even largely replaced, by foreign bodies. This group includes the bath-sponges and their very numerous relations. Families.—Aplysillidae, Spongeliidae, Spongiidae. There are two groups of palaeozoic fossil siliceous sponges which apparently do not fit into the above system, viz. the Octactinellida and Heteractinellida of G. J. Hinde. The former, represented by the genus Astraeospongin, have cctactinal megascleres. The latter, represented by the genera Tholiasterella and Asteractinella, have poly-axon megascleres with an indefinite number of rays. These may indicate the former existence of two distinct classes of siliceous sponge: A .8 (After G. J. Hinds.) which are, so far as we know, totally unrepresented at the present day. C (After G. J. Hinde.) A, Typical polyactine. B, Rosette-like form. C, D, E, Nail-like forms. Phylogeny. The most recent views as to the evolution and inter-relationships of the principal groups of sponges above enumerated may be conveni- 4° ently expressed by the accompanying phylogenetic tree (fig. 42). Starting with the hypothetical Protolynthus as the ancestral form of the entire group, we see how two divergort o `4 lines of descent are very early escab- lished according to whether or not a calcareous skeleton is developed. The Calcarea are at first simple Olynthus forms, Homocoela, differing only from the Protolynthus in the presence of the calcareous spicules. From these are derived, by the process of budding, on the one hand reticulate forms (Clathrina) and on the other radiate forms (e.g. Leucosolenia tripodifera), and some of the latter (now probably extinct) form the starting point for the evolution of the Calcarea Heterocoela, begin- ning with simple Syconoid forms a'and ending with complex Leuconoids, in which the original process of simple budding has been followed up Protolynthue by elaborate modifications of both showing the supposed relation- Turning to the other main line of ships of the principal groups descent we find at once a conspicuous of sponges to one another. gap between the Protolynthus and the simplest known non-calcareous sponge; though the analogy of the Calcarea makes it easy to understand how the almost Syconoid canal system of the simplest Hexactinellids, or the primitive Rhagon type of other groups, may have been derived from the Protolynthus ancestor in the first instance by simple budding. This line of descent may be regarded as continued straight on into the existing Myxospongida, with increase in the complexity of the canal system, due to folding of the chamber- bearing layer and the accompanying development of inhalent and exhalent canal systems, but without the development of any skeleton. The Triaxonida and Euceratosa would seem to have branched off independently at a very early stage from the Myxo- sponge line, before the flagellated chambers had suffered that reduc- tion in size which occurs in some existing Myxospongida and in all Tetraxonida. In the Triaxonid line of descent the evolution of the siliceous skeleton of primitively hexactinellid spicules is the leading feature, the canal system preserving remarkable uniformity through- out the group. In the Tetraxonida also the skeleton has played the principal part in the evolution of existing species, but the canal system too has undergone great modifications. The primitive tetrax- onid, tetractinellid siliceous spicules must have arisen quite indepen- dently, their fundamental form being totally different from that of the triaxonid hexactinellid type. The appearance of differentiated microscleres in this group introduced new possibilities of variation, of which full advantage has been taken, and we are confronted with most interesting evolutionary series, terminating in many very remarkable and at present inexplicable spicule forms (fig. 29). In many of the more advanced Tetraxonida, especially in the Chalininae, the development of spongin cement also appears as a new factor in the process of evolution. At first serving merely to glue the megascleres together into a continuous framework, it ultimately, in some extreme cases, completely replaces the siliceous skeleton and gives rise to a purely " horny " skeleton in which all traces of spicules have been lost by degeneration. Thus we arrive at a " Pseudoceratose " condition (fig. 32, D) which must be carefully distinguished from the condition of the Euceratosa, which have apparently branched off quite independently from Myxosponge ancestors. Here we have another typical example of that phenomenon of " convergence " which has rendered the classification of sponges so very difficult. In the Euceratose line of descent we start with forms (Aplysilla) with large sac-shaped chambers and altogether primitive canal system, accompanied by an arborescent horny skeleton (fig. 33) of an entirely different type from that of the pseudoceratose Tetraxonida. From this we can trece the evolution gradually through the Spongeliidae to the Spongiidae, the skeleton becoming reticulate and the canal system gradually more complex with accompanying reduction in size of the chambers. The bath. sponge perhaps represents the culminating point in this direction. Thus it appears that both the horny type of skeleton and the siliceous spicular type have been twice independently produced in the evolution of the Non-calcarea. An analogous case of convergence is seen in the union of originally separate spicules into a coherent skeleton by means of cement of the same chemical composition as themselves. This has taken place independently in the Calcarea (Petrostoma), the Dictyonine Hexactinellida and the Lithistid Tetraxonida. Affinities of the Porif era. Three main views have been put forward with regard to the position of the Sponges in the animal kingdom: (I) that they are colonies of Protozoa; (2) that they form a subdivision of the Coelenterata; (3) that they are not Protozoa but have originated from Protozoon ancestors quite independently from other Metazoa (Enterozoa). The first of these views, associated especiai'.y with the names of James Clark and Saville Kent, is supported by the relative independence of the constituent cells in the sponge-body and by the extraordinary resemblance of the collared cells to the choanoflagellate or collared Monads. It is also supported by the existence of a remarkable colonial form of Choanoflagellata (Proterospongia) in which the collared Monads are partially embedded in the surface of a gelatinous matrix, in the interior of which amoeboid cells are found. E. A. Minchin has shown that even in the adult Leucosolenia (Clathrina) the collared cells and porocytes have the power of changing their relative positions, while migration of dermal and gastral cells and consequent inversion of the layers appears to be a common feature of the sponge larva at the time of metamorphosis. These facts are certainly suggestive of Protozoon colonies rather than of Metazoa. On the other hand it must not be forgotten that migratory amoebocytes (leucocytes) occur in probably all groups of Metazoa, while the degree of integration and the amount of histological differentiation in Sponges are far greater than in any other Protozoon colonies known to us. It has been argued that the process of sexual reproduction by means of ova and spermatozoa is fatal to the Protozoon-colony theory, but this argument is completely disposed of by the discovery of spermatozoa and ova in the unicellular Sporozoa. On the other hand the occurrence of collared cells has been held to distinguish the Sponges from all other Metazoa, and this argument has also been answered by the discovery of collared cells in the larva of Echinocyamus (an Echinoderm) by H. Theel. It would, in short, , be difficult to frame a definition of the Protozoa which should absolutely exclude the Sponges, while at the same time our conception of the nature of Protozoa will have to be profoundly modified if we are to admit the Sponges within the limits of that group. The second view, that the Sponges constitute a subdivision of the Coelenterata, is maintained by some very eminent continental authors such as Ernst Haeckcl and F. E. Schulze. This view is supported by the structure of the Olynthus type, which, as we have seen, forms the starting-point of Sponge evolution. The dermal layer of the Olynthus is regarded as ectoderm, the gastral layer as endoderm and the mesogloea with its contained cells as mesoderm, more highly developed A B than in most Coelenterates. It is also supported by a consider-able amount of agreement in the early stages of development, up to the formation of the ciliated larva. According to this view the Olynthus, or at any rate the imaginary " Protolynthus" is only a slightly modified gastrula, and the Sponges are there-fore Enterozoa without any coelom, or in other words Coelenterata. The extraordinary histological differences between the Sponges and other Coelenterates (Cnidaria), combined with the highly characteristic canal system and the absence of tentacles, are, however, alone sufficient to throw grave doubts upon the. probability of a close relationship between the two groups, and these doubts are greatly strengthened by recent embryological researches, which tend to show that the so-called ectoderm and endoderm are not homologous in the two cases. There remains the third view, in accordance with which the Sponges are multicellular animals which have originated quite independently from Choanoflagellate Protozoon ancestors, and this is the view which at present seems to have most in its favour. It is especially associated with the name of W. J. Sollas, who invented the term " Parazoa " for the group. In support of this view it may be pointed out that the tendency to form hollow, spherical colonies, resembling the blastosphere stage in the development of Enterozoa, is met with in very distinct groups of Protozoa (e.g. Volvox, Sphaerozoum). This form of colony is obviously polyphyletic in origin. The fact that the segmentation of the ovum leads to such a form in both Sponges and Enterozoa is therefore by no means conclusive evidence that Sponges and Enterozoa have originated from the same Protozoon group. While, as has been repeatedly pointed out, the universal and characteristic collared cells of sponges point emphatically to a Choanoflagellate ancestry, it is impossible, in the present state of our knowledge, to indicate the particular Protozoon group which has given origin to the Enterozoa. We may then consider the Metazoa, or many-celled animals, as a polyphyletic, or at any rate diphyletic group, including two perfectly distinct lines of descent from the ancestral Protozoa, the Sponge-line on the one hand, which leads to nothing higher than Sponges, which retain in many respects the characters of Protozoa, and the Enterozoon line on the other, which leads through the Coelenterata to the Coelomata and so on to the highest divisions of the animal kingdom. Economics. All the bath sponges belong to the two genera Euspongia, Bronn, and Hippospongia, Schulze, subdivisions of the old genus Spongia, auctorum, distinguished from one another by the fact that in Hippospongia the body of the sponge is traversed by wide ramifying canals or vestibules, in addition to the proper canal system of the sponge. Species of these two genera occur in many parts of the world, probably wherever the temperature of the sea-water is sufficiently high and the depth and bottom suitable. It is only in a few localities, however, that they occur in sufficient numbers and of sufficiently good quality to render a sponge fishery practicable. The sponges of commerce are obtained chiefly from the Mediterranean, the coast of Florida and the Bahama Islands. From the Mediterranean three distinct species are obtained—(1) Euspongia ojjicinalis, which includes the " fine sponges," with two chief varieties, mollissima (the Levantine sponges, very soft and often cup-shaped), and adriatica; (2) Euspongia zimocca, including the " hard" or Zimocca sponges; (3) Hippospongia equine, the " common " or " horse " sponge. Of the Florida sponges five principal kinds are recognized by the dealers—(1) the sheep's wool sponge (Hippospongia gossypina)—this appears to be by far the most abundant in the market and also the most valuable; (2) the yellow sponge (Euspongia agaricina), resembling the Zimocca sponge of the Mediterranean; (3) the grass sponges (including both Hippospongia graminea and H. cerebriformis); (4) the velvet sponge Hippospongia maeandriniformis), which is not so common as the others; (5) the glove sponge (Euspongia tubulifera), which is the least valuable. In the year 19oo the Florida spongefisheries yielded 418,125 lb of sponges, valued at $567,685. The Bahama sponges appear to be very similar to those of Florida. Bath sponges occur in comparatively shallow water and are obtained by diving, by dredging, or by means of a trident or long-handled fork. The preparation of the sponges for the market is extremely simple. The slimy soft tissues very soon begin to decay and run off when they are removed from the water; after this has gone on for some time the sponges are washed and beaten until the skeleton is clean, they are then threaded on string and dried. They are frequently " loaded " with foreign matter by the dealers in order to increase their weight; rock-salt, glucose, molasses, lead, gravel, sand and stones being used for the purpose. They are also often bleached by means of chemicals to give them a better colour, but though their appearance is thereby greatly improved, their durability is said to be impaired. In spite of the undoubted rapidity with which sponges grow, as shown by the fact that on the coast of Florida marketable sponges are found commonly in places that had been stripped of saleable specimens in the preceding year, there appears to be considerable danger of injury to the sponge industry by over-fishing and by the reckless destruction of young specimens, and it has been found necessary to introduce special legislation in America to counteract these evil tendencies. The question of the artificial propagation and cultivation of sponges has also been much discussed, but although some very interesting experiments have been made, they have not as yet led to any great practical results. As far back as 1862 Oscar Schmidt showed that " cuttings " of sponges will attach themselves and grow. This idea was followed out in the experiments of G. Buccich on the Island of Lesina, from 1863-1872, but these experiments were brought to a close by the hostility of the native fishermen. Sifnilar experiments have since been made on the Florida sponge-grounds. The possibility of rearing sponges in this way from cuttings has thus been fully demonstrated, but whether it can be done profitably is another question. According to the experience of G. Buccich it appeared that it would take seven years for the cuttings to attain marketable size in the Mediterranean. The Florida experiments, on the other hand, indicate a much more rapid rate of growth, and it has been stated that under favourable conditions the cuttings will attain marketable size in as short a time as one year. It has been doubted, however, whether the total weight of sponges produced by cuttings would be greater than the weight of the sponges from which the cuttings were taken if these sponges were allowed to continue their growth undisturbed. H. V. Wilson has suggested that sponges may be artificially reared from the eggs, in the same way that fishes or oysters are reared. The eggs of the bath sponge, like those of other sponges, develop into free-swimming ciliated larvae, and these might be made to attach themselves, like oyster-spat, to suitable objects, on which the young sponges could be cultivated under appropriate conditions. Detailed experiments are required to demonstrate the feasibility or otherwise of this interesting suggestion. For further information on the economic aspect of the subject the student should consult the annual Bulletin and special papers of the United States Bureau of Fisheries and also the work of Seurat referred to in the bibliography. (1) J. S. Bowerbank, Monograph of the British Spongiadae (Ray Society) ; (2) H. J. Carter, a long series of memoirs, chiefly systematic, in the Annals and Magazine of Natural History (1847 to 1887); (3) Y. Delage, Embryogenie des eponges (Arch. Zool. Exp. (2), x. 1892) ; (4) A. Dendy, ' Monograph of Victorian Sponges," pt. i., Trans. Royal Soc., Victoria III. (1891) ; (5) idem, " Studies on the Comparative Anatomy of Sponges," pts. Levi., Quart. Journ. Mic. Sci. (1888–1894) ; (6) idem, " Report on the Sponges collected by Professor Herdman at Ceylon in 1902 " (Royal Society, 1905); (7) E. Haeckel, Die Kalkschwamme (Berlin, 1872); (8) G. J. Hinde, Monograph of British Fossil Sponges (Palaeontological Society, London) ; (9) A. Hyatt, " Revision of the North American Poriferae," Mem. Boston Soc. Nat. Hist. (1875–1877), vol. ii.; (to) R. Kirkpatrick, Descriptions of South African Sponges (Marine Investigations in South Africa; Cape of Good Hope Department of Agriculture, 1902–1903); (II) W. Lundbeck, Porifera (Danish Ingolf-Expedition, vol. vi., 1902, &C.); (12) E. A. Minchin " Materials for a Monograph of the Astons," I., Quart. Journ. Mic. Sci. (1898), vol. xl.; (13) idem, " Sponges," in Lankester's Zoology, pt. ii. (1900); (14) N. Polejaeff, " Calcarea," " Challenger " Reports, ' Zoology" (1883), vol. viii.; (15) S. 0. Ridley and A. Dendy, " Monaxonida," " Challenger" Reports, " Zoology " (1888) vol. xx.; (16) F. E. Schulze, " Untersuchungen abet- den Bau and die Entwicklung der Spongien," Zeitschrift fur wiss. Zoologie (1875–1881) ; (17) idem, " Hexactinellida" (" Challenger " Reports, "Zoology," vol. xxi.); (IS) idem, Amerikanische Hexactinelliden (Gustav Fischer, Jena, 1899); (19) idem, Hexactinellida of the " Valdivia" Expedition (Jena, 1904) ; (20) L. G. Seurat, L'Eponge histoire naturelle; Peche; " Acclimatation "; Spongiculture, Bull. SOC. nat. d'acclimatation de France, 48th year (1901); (21) W. J. Sollas, " Tetractinellida " (" Challenger" Reports, " Zoology,"' vol. xxv.) • (22) I. B. J. Sollas, " Sponges," Cambridge Natural History (1906), vol. i.; (23) E. Topsent, Etudes monographiques des spongiaires de France (Arch. Zool. Exp. (3), 1894, vol. ii. &c.); (24) idem, " Contribution A. l'etude des spongiaires de I'Atlantique Nord " (Campagnes scientifiques du prince de Monaco, 1892, vol. u.) ; (25) idem, " Spongiaires des Acores " (Campagnes scientifiques du prince de Monaco, 1904, vol. xxv.) ; (26) G. C. J. Vosmaer, Spongien," Bronn's Klassen and Ordnungen des Thierreichs (1887), vol. ii.; (27) H. V. Wilson, " On the Feasibility of Raising Sponges from the Egg," Bulletin of the United States Fish Commission (1897), vol. xvii. (A. DE.)
End of Article: SPONGES
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