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Originally appearing in Volume V14, Page 147 of the 1911 Encyclopedia Britannica.
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HYDROMEDUSAE, a group of marine animals, recognized as belonging to the Hydrozoa (q.v.)by the followingcharatters. (I) The polyp (hydropolyp) is of simple structure, typically much longer than broad, without ectodermal oesophagus or mesenteries, such as are seen in the anthopolyp (see article AxrHozoA) the mouth is usually raised above the peristome on a short conical elevation or hypostome; the ectoderm is without cilia. (2) With very few exceptions, the polyp is not the only type of individual that occurs, but alternates in the life-cycle of a given species, with a distinct type, the medusa (q.v.), while in other cases the polyp-stage may be absent altogether, so that only medusa-individuals occur in the life-cycle. The Hydromedusae represent, therefore; a sub-class of the Hydrozoa. The only other sub-class is the Seyphomedusae (q.v.). The Hydromedusae contrast with the Scyphomedusae in the following points. (I) The polyp, when present, is without the strongly developed longitudinal retractor muscles, forming ridges (taeniolae) projecting into the digestive cavity, seen in the scyphistoma or scyphopolyp. (2) The medusa, when present, has a velum and is hence said to be craspedote; the nervous system forms two continuous rings running above and below the velum; the margin of the umbrella is' not lobed (except in Narcomedusae) but entire; there are characteristic differences in the sense-organs (see below, and SCYPHOMEDUSAE) ; and gastral filaments (phacellae), subgenital pits, &c., are absent, (3) The gonads, • whether formed in the polyp or the medusa, are developed in the ectoderm. The Hydromedusae form a widespread, dominant and-'highly differentiated group of animals, typically marine, and 'found in all seas and in all zones of marine life. F1'ek,h-water forms, however, are also known, very few as regards''speeiss" or genera, but often extremely abundant as individuals. I1f• the British fresh-water fauna only two genera, Hydra airs Cor,dylophora, are found; in America occurs an additional getl$, Microhydra. The paucity of fresh-water forms contrasts shar$ly with the great abundance of marine genera common in all sea's, and on every shore. The species of Hydra, however, are extremely common and familiar inhabitants of ponds and ditches. In fresh-water Hydromedusae the life-cycle i ,usually secondarily simplified, but in marine forms the lift-cycle may be extremely complicated, and a given species often passes in the course of its history through widely different forma adapted to different habitats and modes of life. Apart from 'k€ral : or embryonic forms there are found typically tw`o types of person, as already stated, the polyp and the medusa; each of which may vary independently of the other, since their envrotl`ment and life-conditions are usually quite different. Hence both polyp, and medusa present characters for classification, and a given species, genus or other taxonomic category may be defined by polyp-characters or medusa-characters or by both combined. If our knowledge of the life-histories of these organisms. were perfect, their polymorphism would present no difficulties to classification; but unfortunately this is far from being the case. In the majority of cases we do not know the polyp corresponding to a given medusa, or the medusa that arises from a given polyp.' Even when a medusa is seen to be budded from a polyp under observation in an aquarium; the difficulty is not always solved, since the freshly-liberated, immature medusa may differ greatly from the full-grown, sexually-mature medusa after several months of life on the high seas (see figs. u, B,C, and 59, a, b, c). To establish the exact relationship it is necessary not only to breed but to rear the medusa, which cannot always be done in 1 In some cases hydroids have been reared in aquaria from ova of medusae, but' these hydroids have not yet been found in the sea (Browne [to al). (9) (1o) (12) (13), (14) (15) (t6) confinement. The alternative is to fish all stages of the medusa in its growth in the open sea, a slow and laborious method in which the chance of error is very great, unless the series of stages is very complete; At present, therefore, classifications of the Hydromedusae have a more or less tentative character, and are liable to revision with increased knowledge of the life-histories of these organisms. Many groups bear at present two names, the one representing the group as defined by polyp-characters, the other as defined by medusa-characters. It is not even possible in all cases to be certain that the polyp-group corresponds exactly to the medusa-group, especially in minor systematic categories, such as families. The following is the main outline of the classification that is adopted in the present article. Groups founded on polyp-characters are printed in ordinary type, those founded on medusa-characters in italics. For definitions of the groups see below. Sub-class Hydromedusae (Hydrozoa Craspedota). Order I. Eleutheroblastea. , II. Hydroidea (Leptolinae). Sub-order I. Gymnoblastea (Anthomedusae). 2. Calyptoblastea (Leptomedusae). Order III. Hydrocorallinae. „ IV. Graptolitoidea. V. Trachylinae. Sub-order r. Traehomedusae. 2. Narcomedusae. Order VI. Siphonophora. Sub-order I. Chondrophorida. 2. Calycophorida. 3. Physophorida. 4. Cystophorida. Organization and Morphology of the Hydromedusae. As already stated, there occur in the Hydromedusae two distinct types of person, the polyp and the medusa; and either of them is capable of non-sexual reproduction by budding, a process which may lead to the formation of colonies, composed of more or fewer individuals combined and connected together. The morphology of the group thus falls naturally into four sections—(r) the hydropolyp, (2) the polyp-colony, (3) the hydro-medusa, (4) the medusa-colonies. Since, however, medusa-colonies occur only in one group, the Siphonophora, and divergent- views are held with regard to the morphological interpretation of the members of a siphonophore, only the first three of the above sub-divisions of hydromedusa morphology will be dealt with here in a general way, and the morphology of the Siphonophora will be considered under the heading of the group itself. r. The Hydropolyp (fig. I)—The general characters of this organism are described above and in the articles HYDROZOA and Pony's. It is rarely free, but usually fixed and incapable of locomotion. The foot by which it is attached often sends out root-like processes—the hydro- rhiza (c). The column (b) is generally long, slender and stalk- like (hydrocaulus). Just below the crown of tentacles, however, the body widens out to form a " head,” termed the hydranth (a), containing a stomach-like dilatation of the digestive cavity. On the upper face of the hydranth the crown of tentacles (t) surrounds the peristome, from which rises the conical hypostome, bearing the mouth at its extremity. The general ectoderm covering the surface of the body has entirely lost the cilia present in the earlier larval stages (planula), and may be naked, or clothed in a cuticle or exo- skeleton, the perisarc (ps), which in its simplest condition is a chitinous membrane secreted by the ectoderm. The perisarc when present invests the hydrorhiza and hydrocaulus; it may stop short below the hydranth, or it may extend farther. In general there are two types of exoskeleton, characteristic of the two principal divisions of the Hydroidea. In the Gymnoblastea the perisarc either stops below the hydranth, or, if continued on to it, forms a closely-fitting investment extending as a thin cuticle as far as the bases of the tentacles (e.g. Bimeria, see G. J. Allman [ii,i pl. xii. figs. r and 3). In the Calyptoblastea the pensarc is always continued above the From Allman's Gymn oblsstic Hydroids, by permiccion of the Council of the Ray Society. hydrocaulus, and forms a cup, the hydrangium or hydrotheca (h, t), standing off from the body, into which the hydranth can be retracted for shelter and protection. The architecture of the hydropolyp, simple though it be, furnishes a long series of variations affecting each part of the body. The greatest variation, however, is seen in the tentacles. As regards number, we find in the aberrant forms Protohydra and Microhydra tentacles entirely absent. In the curious hydroid Monobrachium tentacle is present, and the same is the case in Clathrozoon; in Amphibrachium and in Lar (fig. II, A) the polyp bears two tentacles only. The reduction of the tentacles in all these forms may be correlated with their mode of life, and especially with living in a constant current of water, which brings food-particles always from one direction and renders a complete whorl or circle of tentacles unnecessary. Thus Microhydra lives amongst Bryozoa, and appears to utilize the currents produced by these animals. Protohydra occurs in oyster-banks and Monobrachium also grows on the shells of bivalves, and both these hydroids probably fish in the currents produced by the lamellibranchs. Amphibrachium grows in the tissues of a sponge, Euplectella, and protrudes its hydranth into the canal-system of the sponge; and Lar grows on the tubes of the worm Sabella. With the exception of these forms, reduced for the most part in correlation with a semi-parasitic mode of life, the tentacles are usually numerous. FIG. 3.—Diagram of It is rare to find in the polyp a regular, Corymor phis. A, A hydrisymmetrical disposition of the tentacles form person giving rise as in the medusa. The primitive number to medusiform persons of four in a whorl is seen, however, in by budding from the Stauridium (fig. 2? ' and Cladonema margin of the disk; B, (Allman [I], pl. xvii.), and in Clavatella free swimming medusa each whorl consists regularly of - eight Forbes) (Allman, loc. cit. pl. xviii.). As a rule, detached ro of the same, however, the number in a whorl is deached from the same, irregular. The tentacles may form a twith omedus e) and dionlyy lly single whorl, or more than one; thus in Corymorpha (fig. 3) and Tubular= one ne tentacle. and (After All-(fig. 4) there are two circlets; in Staur- man). idsum (fig. 2) several; in Coryne and Cordylophora the tentacles are scattered irregularly over the elongated hydranth. As regards form, the tentacles show a number of types, of which the most important are (I) filiform, i.e. cylindrical or tapering from 'The numbers in square brackets [ ] refer to the bibliography at the end of this article; but when the number is preceded by the word Hydrozoa, it refers to the bibliography at the end of the article HYDROZOA. „ „ a, Hydranth; b, Hydrocaulus; c, Hydrorhiza; t, Tentacle; ps, Perisarc, forming in the region of the hydranth a cup or hydrotheca(h, t), —which, however,is only found in polyps of the order Calyptoblastea. a single base to extremity, as in Clava (fig. 5); (2) capitate, i.e. knobbed at the extremity, as in Coryne (see Allman, loc. cit. p1. iv.); (3) branched, a rare form in the polyp, but seen in Cladocoryne (see Allman, loc. cit. p. 38o, fig. 82). Sometimes more than one type of form is found in the same polyp; in Pennaria and Stauridium (fig. 2) the upper whorls are capitate, the lower filiform. Finally, as regards structure, the tentacles may retain their primitive hollow nature, or become solid by obliteration of the axial cavity. The hypostome of the hydropolyp may be small, or, on the other hand, as in Eudendrium (Allman, loc. cit. pis. xiii., xiv.), large and trumpet - shaped. In the curious polyp Myriothela the body of the polyp is differentiated into nutritive and reproductive portions. Histology. — The ectoderm of the hydropolyp is chiefly sensory, contractile and protective in function. It may also be glandular in places. It consists of two regions, an external epithelial layer. and a more internal sub-epithelial layer. The epithelial layer consists of (I) so-called " indifferent " cells secreting the perisare or cuticle and modified to form glandular cells in places; for example, the adhesive cells in the foot. (2) Sensory cells, which may be fairly numerous in places, especially on the tentacles, but which occur always scattered and isolated, never aggregated to form sense-organs as in the medusa. (3) Contractile From Allman's Gynasobiastic Hydroids, by permission of the Council of the Ray Society. or myo-epithelial cells, with the cell prolonged at the base into a contractile muscle-fibre (fig. 6, B). In the hydropolyp the ectodermal muscle-fibres are always directed longitudinally. Belonging primarily to the epithelial layer, the muscular cells may become secondarily sub-epithelial. The sub-epithelial layer consists primarily of the so-called interstitial cells, lodged between the narrowed basal portions of the epithelial cells. From them are developed two distinct types of histological elements; the genital cells and the cnidoblasts or mother-cells of the nematocysts. The sub-epithelial layer thus primarily constituted may be recruited by immigration from without of other elements, more especially by nervous (ganglion) cells and rntisclecells derived from the epithelial layer. In its fullest development, therefore, the sub-epithelial layer consists of four classes of cellelaments. The genital cells are simple wandering cells (archaeocytes), at first minute and without any specially distinctive features, until they begin to develop into germ-cells. According to Wulfert [6o] the primitive germ-cells of Gonoihyraea can be distinguished soon after the fixation of the planula, appearing amongst the interstitial cells of the ectoderm. The germ-cells are capable of extensive migrations, not only in the body of the same polyp, but also from parent to bud through many non-sexual generations of polyps in a colony (A. Weismann [58]). The cnidoblasts are the mother-cells of the nematocysts, each' cell producing one nematocyst in its interior. The complete nematocyst (fig. 7) is a spherical or oval capsule containing a hollow thread, usually barbed, coiled in its interior. The capsule has a double wall, an outer one (o.c.), tough and rigid in nature, and an inner one (i.c.) of more flexible consistence. The outer wall of the capsule is in-complete at one pole, leaving an aperture through which the thread is discharged. The inner membrane is continuous with the wall of the hollow thread at a spot immediately below the aperture in the outer wall, so that the thread itself (f) is simply a hollow prolongation of the wall of the inner capsule inverted and pushed into its cavity. The entire nematocyst is enclosed in the cnidoblast which form d it. When the nematocyst is completely developed, the cnidobl4st passes outwards so as to occupy a superficial position in the ectoderm, and a delicate protoplasmic process of sensory nature, termed the cnidocil (cn) projects from the cnidoblast like a fine hair or ciliufn. Many points in the development and mechanism of the nematocyst are disputed, but it is tolerably certain (i) that the cnidocil is of sensory nature, and that stimulation, by contact with prey or in other ways, causes a reflex discharge of the nematocyst; (2) that the discharge is an explosive change whereby the in-turned thread is suddenly everted and turned inside out, being thus shot through the opening in the outer wall of the capsule, and forced violently Into the tissues of the prey, or, it may be, of an enemy; (3) that the thread inflicts not merely a mechanical wound, but instils an irritant poison, numbing and paralysing in its action. The points most in dispute are, first, how the explosive discharge is brought about, whether by pressure exerted external to the capsule (i.e. by contraction of the cnidoblast) or by internal pressure. N. Iwanzov [271 has brought forward strong grounds for the latter view, pointing out that the cnidoblast has no contractile mechanism and that measurements show discharged capsules to be on the average slightly larger than undischarged ones. He believes that the capsule contains a sub-stance which swells very rapidly when brought into contact with water, and that in the undischarged condition the capsule has its opening closed by a plug of protoplasm (x, fig. 7) which prevents rh- access of water to the contents; when the enidocil is'stimulated it sets in action a mechanism or perhaps a series of chemical changes by which the plug is dissolved or removed; as a result water penetrates into the capsule and causes its contents to swell, with the result that the thread is everted violently. A second point of dispute concerns the spot at which the poison is lodged. Iwanzov believes it to be contained within the -thread itself before discharge, and to. be introduced into the tissues of the prey by' the eversion of the thread. A third point of dispute is whether the nematocysts are formed in situ, or whether the cnidoblattts migrate with them to the region where they' are most needed; the fact that in Hydra, for example, there are no interstitial cells in the tentacles, where nematocysts are very abundant, is certainly in favour of the view. that the cnidoblasts migrate on to the tentacles from the body, and that like the genital cells the cnidoblasts are wandering cells. The muscular tissue consists primarily of processesfrom the bases of the epithelial cells, processes which are contractile in nature and may be distinctly striated. A a, Undischarged nematocyst. b, Commencing discharge. c, Discharge complete. forming a sub-epithelial cn, Cnidocil. contractile layer, de_ N, Nucleus of cnidoblast. veloped chiefly tn the ten- o.c, Outer capsule. tacles of the polyp. The x, Plug closing the opening of the evolution of the ganglion- outer capsule. cells, is probably similar; i.c., Inner capsule, continuous with the an epithelial cell develops wall of the filament, f. processes of nervous nature b, Barbs. from the base; which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells; next the nerve-cell loses its connexion with the outer epithelium and becomes a sub-epithelial ganglion-cell which is closely connected with the muscular layer, conveying stimuli from the sensory cells to the contractile elements. The ganglion-cells of Hydromedusae are generally very small. In the polyp the nervous tissue is always in the form' of a scattered plexus, never con- centrated to form a definite nervous system as in the medusa. The endoderm of the polyp is typically a flagellated epithelium of large cells (fig. 6), from the bases of which arise contractile muscular processes lying in the plane of the transverse section of the body. In different parts of the coelenteron the endoderm' may be of three principal types-(I) digestive endoderm, the primi- tive type, with cells of large size and considerably vacuolated, found in the hydranth; sortie of these cells may become special glandular cells, without flagella' or contractile processes; (2) circulatory endoderm, without vacuoles and without basal contractile processes, found in the hydrorhiza and hydrocaulus; (3) supporting endoderm (fig. 8), seen in solid tentacles as a row of cubical vacuolated cells, occupying the axis of the tentacle, greatly resembling notochordal tissue, particularly that of Arnphioxus at a certain stage of development; as a fourth variety of endodermal cells excretory cells should perhaps be reckoned, as seen in the pores in the foot of Hydra and elsewhere (cf. C. Chun, HYDROZOA PP. 314, 315)- The mesogioea in the hydropolyp is a thin elastic layer, in whichmaybe lodged the muscular fibres and ganglion cells mentioned above. but which never contains any connective tissue or skeletogenous cells or any other kind of special mesogloeal corpuscles. 2. The Polyp-colony.---All known hydropolyps possess the power of reproduction by budding, and the buds produced may become either polyps or medusae. The buds may all be-come detached after a time and give rise to separate and in-dependent individuals, as in the common Hydra, in which only polyp-individuals are produced and sexual elements are developed upon the polyps themselves; or, on the other hand, the polyp individuals produced by budding may remain permanently in connexion with the parent polyp, in which case sexual elements are never developed on polyp-individuals but only on medusa-individuals, and a true colony is formed. Thus the typical hydroid colony starts from a " founder polyp, which in the vast majority of cases is fixed, but which may be floating, as in Nemopsis, Pelagohydra, &c. The founder-polyp usually produces by budditlg polyp-individuals, and these in th,pir turn produce other buds. The polyps are all non-sexual individuals whose function is purely nutritive. After a time the polyps, or certain of them, produce by budding medusa-individuals, which sooner or later develop sexual elements; in some cases, however, the founder.. polyp remains solitary, that is to say, does not produce polyp-buds, but only medusa-buds, from the first (Corymorpha, fig. 3, Myriothela, &c.). In primitive forms the medusa-individuals are set free before reaching sexual maturity and do not con-tribute anything to the colony. In other cases, however, the medusa-individuals become sexually mature while still attached to the parent polyp, and are then not set free at all, but become appanages of the hydroid colony and undergo degenerative changes leading to reduction and even to complete obliteration of their original medusan structure. I n this way the hydroid colony becomes composed of two portions of different function, the nutritive " trophosome," composed of non-sexual polyps, and the reproductive "gonosome," composed of sexual medusa-individuals, which never exercise a nutritive function while attached to the colony. As a general rule polyp-buds are produced from the hydrorhiza and hydrocaulus, while medusa-buds are formed on the hydranth. In some cases, however, medusa= buds are formed on the hydrorhiza, as in Hydrocorallines. In such a colony of connected individuals, the exact limits of the separate " persons " are not always clearly marked out. Hence it is necessary to distinguish between,first,the "zooids," indicated in the case of the polyps by the hydranths, each with mouth andtentacles; and, secondly, the "coenosarc, or common flesh, which cannot be assigned more to one individual than another, but consists of a more or less complicated network of tubes, corresponding to the hydrocaulus and hydrorhiza of the primitive independent polypp--individuaI. fihe coenosarc constitutes a system by which the digestive Cavity of any one polyp is put into communication with that of any other individual either of the trophosome or gonosdme. In this manner the food absorbed by one individual contributes to the welfare of the whole ' colony, and the coenosarc has the the epithelium and come to lie entirely beneath it, From Gegenbaur's Elements of Cost-}arative Anatomy. Fia. 8.— Vacuolated Endoderm Cells of cartilaginous consistence from the axis of the tentacle of a Medusa (Canino). From Allman's Gymnoblastic Hydroids, by permission of the Council of the Ray Society. ' From Allman's Gymnoblastie by permission of the Council of Society. Hydroids, the Ray theoretically, of unlimited growth in a vertical direction, and as it grows up it throws out buds tight and left alternately, so that the first bud produced by it is the lowest down, t he second bud is above the first, the third above this again, and so on. Each bud produced function of circulating and distributing nutriment through the colony. The hydroid colony shows many variations in form and architecture which depend simply upon differences in the methods in which polyps are budded. In the first place, buds may be produced only from the hydrorhiza, which grows out and branches to form a basal stolon, typically net-like, spreading over the substratum to which the founder-polyp attached itself. From the stolon the daughter-polyps grow up vertically. The result is a spreading or creeping colony, with the coenosarc in the form of a root-like horizontal network (fig. 5, B; II, A). Such a colony may undergo two principal modifications. The meshes of the basal network may become very small or virtually obliterated, so that the coenosarc be-comes a crust of tubes tendingtofusetogether, and covered over by a common perisarc. Encrusting colonies of this kind are seen in Clava squamata (fig. 5, A) and Hydractitria (figs. 9, 1o), the latter having the perisarc A calcified. A further very important modifi- After Hincks, Forbes, and Browne. A and B modified cation is seen when the from Hincks; C modified from Forbes's Brit. Naked-eyed Med tubes of the basal urae. perisarc do not remain of its Medusa, Willia stellata. A, colony of but grow in all planes Las; B and C, young and adult medusae. forming a felt-work; the result is a massive colony, such as is seen in the so-called Hydrocorallines (fig. 6o), where the interspaces between the coenosatml tubes are filled tip with calcareous matter, or caenosteum, replacing the chitinous perisarc. The result is a stony, solid mass, which contributes to the building up of coral reefs. In massive colonies of this kind no sharp distinction can be drawn between hydrorhiza and hydro- Y.caulus in the ~ooeeosarc; it is practically all hydrorhiza. Massive colonies may assume es- various forms and are often branching or tree-like. A fur- - they peculiarity of this type of colony is that the entire coeno- sarcal complex is covered externally by a common layer of ectoderm ; it is not clear how this cohering layer is developed. In the second place, the buds may be produced from the hydrocaulus, growing out laterally from it; the result is an arborescent, tree-like colony (figs. 12, 13). Budding from the hydrocaulus may be combined with budding from the hydrorhiza, so that numerous branching colonies arise from a common basal stolon. In the formation of arborescent colonies, two sharply distinct types of budding are found, which are best deseribed in botanical terminology as the monopodial or racemose, and the sympodial or cymose types respectivell, ; each is characteristic of one of the two sub-orders of the Hydrrndea, the Gynlnoblastea and Calyptoblastea. In the monopodial method (figs.' i2, 14) the founder-polyp is, by the founder proceeds to grow and to bud in the. same way as the founder did, producing a side branch of the main stem. Hence, in a colony of gymnoblastic hydroids, the oldest polyp of each system, that is to say, of the main stem or of a branch, is the topmost polyp; the youngest polyp of the system is the one nearest to the topmost polyp; and the axis of the system is a true axis. In the sympodial method of budding, on the other hand, the founder-polyp is of limited growth, and forms a bud from its side, which is also of limited growth, and forms a bud in its turn, and so on (figs. i5, 16). Hence, in a colony of calyptoblastic hydroids, the oldest polyp of a system is the lowest; the youngest polyp is the top- most one; and the axis of the system is a false axis composed of portions of each of the consecutive polyps. In this method of budding there are two types.' In one, the biserial type (fig. 15) ,the polyps pro- duce buds right -b3 and left alter- nately, so that the hydranths are arranged in a zig- zag fashion, form- ing a " scorpioid cyrim," as in Obelia and Serlularia. In the other, the uni- serial type (fig.'6), the buds are formed always on 3 1 5 the same side, forming a " heli- serial arrangement becomes masked later by secondary torsions of the hydranths. In a colony formed by sympodial budding, a polyp always produces first a bud, which contributes to the system to which it belongs, i.e. continues the stem or branch of which its parent forms a part. The F polyp may then form a second d bud, which becomes the starting point of a new system, the j t beginning, that is, of a new 4`\ branch; and even a third bud, I starting yet another system, may be produced from the same polyp. Hence the colonies of Calyptoblastea may be coinplexly branched, and the bud-ding may be biserial through-out, uniserial ,throughout, or partly one, partly the other. Z b 4 Thus in Plumularidae (figs. 17, biserial sbudding; eachnpolyp budding, uniserial type, shown on the main stem forms a in four stages (1-4). F, founder- second' bud, which usually polyp; I, 2, 3, succession of polyps forms a side branch or pinnule budded from the founder. by uniserial budding. In this way are formed the familiar feathery colonies of Plumularia, in which the pinnules are. all in one plane, while in the allied Anlennularia the pinnules are arranged in whorls round the main biserial stem. The pinnules never branch again, since in the uniserial mode of budding a polyp never forms a second polyp-bud. On the other hand, a polyp on the main stem may form a second bud which, instead of forming a pinnule by uniserial budding, produces by biserial bud-ding a branch, from which pinnules arise as from the main stem (fig. 18—3, 6). Or a polyp on the main stem, after having budded a second time to form a pinnule, may give rise to a third bud, which starts a new biserial in an interesting manner by H. Driesch [131, to whose memoirs the reader must be referred for further details. Individualization of Polyp-Colonies.—As in other cases where animal colonies are formed by organic union of separate individuals, there is ever a tendency for the polyp-colony as a whole to act as asingle individual, and for the members to become subordinated to the needs of the colony and to undergo specialization for particular functions, with the result that they simulate organs and their individuality becomes masked to a greater or less degree. Perhaps the earliest of such specializations is connected with the reproductive function. Whereas primitively any polyp in a colony may produce medusa-buds, in many hydroid colonies medusae are budded only by certain polyps termed blastostyles (fig. to, b). At first not differing in any way from other polyps (fig. 5), the blastostyles gradually lose their nutritive function and the organs connected with it; the mouth and tentacles disappear, and the blastostyle obtains the nutriment necessary for its activity by way of the coenosarc. In the Calyptoblastea, where the polyps are protected by special capsules of the perisarc, the gonothecae en-closing the blastostyles differ from the hydro-thecae protecting the hydranths (fig. 54). In other colonies the two functions of the nutritive polyp, namely, capture and digestion of food, may be shared between different polyps (fig. 1o). One class FIG. 18.—Diagram showing method of polyps, the dactylozoids of branching in the Plumularia-type; (dz), lose their mouth and compare with fig. 17. Polyps 3 and 6, stomach, and become elon- instead of producing uniserial pinnules, gated and tentacle-like, have produced biserial branches (3', 32, showing great activity of 3', 34; 61-63), which give off uniserial movement. Another class, branches in their turn. the gastrozoids (gz), have the tentacles reduced or absent, but have the mouth and stomach enlarged. The dactylozoids capture food and pass it on to the gastrozoids, which swallow and digest it. Besides the three types of individual above mentioned, there are other appendages of hydroid colonies, of which the individuality is doubtful. Such are the " guard-polyps " (machopolyps) of Plumularidae, which are often regarded as individuals of the nature of dactylozoids, but from a study of the mode of budding in this hydroid family Driesch concluded that the guard-polyps were not true polyp-individuals, although each is enclosed in a small protecting cup of the perisarc, known as a nematophore. Again, the spines arising from the basal crust of Podocoryne have been interpreted by some authors as reduced polyps. 3. The Medusa. —In the Hydromedusae the medusa-individual occurs, as already stated, in one of two conditions, either as an independent organism leading a true life in the open seas, or as a subordinate individuality in the hydroid colony, from which it is never set free; it then becomes a mere reproductive appendage or gono- phore, losing sue FIG. 19.—Diagram showing method of branchcessively its organs ing in the Aglaophenia-type. Polyp 7 has pro-of sense, loco- duced as its first bud, 8; as its second bud, al, motion and nutri- which starts a uniserial pinnule; and as a third tion, until its bud I', which starts a biserial branch (IP-VP) medusoid nature that repeats the structure of the main stem and and organization gives off pinnules. The main stem is indicated become scarcely by –•–•-•, the new stem by • • • • • . recognizable. Hence. it is convenient to consider the morphology of the medusa from these two aspects. (a) The Medusa as an Independent Organism.--The general structure and characteristics of the medusa are described elsewhere (see articles HYDROZOA and MEDUSA), and it is only necessary here to deal with the peculiarities of the Hydromedusa. As regards habit of life the vast majority of Hydromedusae are tt pelagic organisms, floating on the surface of the open sea, propelling I themselves feebly by the pumping movements of the umbrella produced by contraction of the sub-umbral musculature, and capturing their prey with their tentacles. The genera Cladonema (fig. 20) and Clava- tella (fig. 21), how- ever,are ambulatory, creeping forms,living in rock-pools and walking, as it were, on the tips of the proximal branches of each of the tentacles, while the remaining branches serve for capture of food. Cladonema still has the typical medusan structure, and is able to swim about, but in Clavatella the um- brella is so much re- duced that swimming is no longer possible. The remarkable medusa Mnestra parasites isecto-para- sitic throughout life on the pelagic mollusc Phyllirrhoe, attached to it by the sub- umbral surface, and its tentacles have become rudimentary or absent. It is inter- esting to note that Mnestra has been shown by J. W. Fewkes [15] and R. T. Gunther [191 to belong to the same family (Cladone- midae) as Cladonema and Clavatella, and it is reasonable to suppose that the non-parasitic ancestor of Mnestra was, like the other two genera, an ambulatory medusa which acquired louse-like habits. In some ,.t species of the genus Cunina(Narcomedusae) the youngest individuals (actinulae) are parasitic on other medusae (see below), but in later life the parasitic habit is abandoned. No other instances are known of sessile habit in Hydromedusae. The external form of the Hydromedusae varies from that of a deep bell or thimble, characteristic of the Anthomedusae, to the shallow saucer-like form characteristic of the Leptomedusae. It is usual for the umbrella to have an even, circular, uninterrupted margin; but in the order Narcomedusae secondary down-growths between the tentacles produce a lobed, indented margin to the umbrella. The marginal tentacles are rarely absent in non-parasitic forms, and are typically four in number, cor- responding to the four perradii marked by the radial canals. Interradial ten- tacles may be also developed, so that the total number present may be in- creased to eight or to an indefinitely large number. In Willia, Geryonia, &c., however, the tentacles and radial canals are on the plan of six instead of four (figs. it and 26). On the other hand, in some cases the tentacles are less in number than the perradii; in Corymorpha (figs. 3 and 22) there is but a single tentacle, while two are found in Ampphinema and Gemmaria (An- thomedusae), and in Solmundella bitentaculata (fig. 67) and Aeginopsis hensenii (fig. 23) (Narcomedusae). The tentacles also vary considerably in other ways than in number: first, in form, being usually simple, with a basal bulb, but in Cladonem- idae they are branched, often in complicated fashion; secondly, in grouping, being usually given off singly, and at regular intervals from the margin of the umbrella, but in Margelidae and in some Trachomedusae they are given off in tufts or bunches (fig. 24); thirdly. in position and origin, being usually implanted on the extreme edge of the umbrella, but in Narcomedusae they become secondarily shifted and are given off high up on the ex-umbrella (figs. 23 and 25) ; and, fourthly, in structure, being hollow or solid, as in the polyp. In some medusae, for instance, the remarkable deep-sea family Pectyllidae, the tentacles may bear suckers, by which the animal may attach itself temporarily. It should be mentioned t finally that the tentacles are very contractile and extensible, and may therefore present themselves, in one and the same Individual, as long, drawn-out threads, or in the form of short corkscrew - like ringlets; they may stream downwards from the sub-umbrella, or be held out horizontally, or be directed upwards over the ex-umbrella (fig. 23). Each species of After O, Maas, Die eraspedoten Medusen der Plankton Expedition, by permission of Lipsius and Tischer. medusa usually has a characteristic method of carrying its tentacles. The sub-umbrella invariably shows a velum as an inwardly projecting ridge or rim at its margin, within the circle of tentacles; hence the medusae of this sub-class are termed craspedote. The manubrium is absent altogether in the fresh-water medusa Limnocnida, in which the diameter of the mouth exceeds half that of the umbrella; on the other hand, the manubrium may attain a great length, owing to the centre of the sub-umbrella with the stomach being drawn into it, as it were, to form a long proboscis, as in Geryonia. The mouth may be a simple, circular pore at the extremity of the manubrium, or by folding of the edges it may become square or shaped like a Maltese cross, with four corners and four lips. The corners of the mouth may then be drawn out into lobes or lappets, which may have a branched or fringed outline (fig. 27), and in Margelidae the subdivisions of the fringe simulate tentacles (fig. 24). The internal anatomy of the Hydromedusae shows numerous variations. The stomach may be altogether lodged in the manubrium, from which the radial canals then take origin directly as in Geryonia (Trachomedusae) ; it may be with or without gastric pouches. The radial canals may be simple or branched, primarily four, rarely six in number. The ring-canal is drawn out in Narcomedusae into festoons corresponding with the lobes of the margin, and may be obliterated altogether (Samaria). In this order the radial canals are represented only by wide gastric pouches, and in the family Solmaridae are suppressed altogether, so that the tentacles and the festoons of the ring-canal arise directly from the stomach. In Geryonia, centripetal canals, ending blindly, arise from the ring-canal and run in a radial direction towards the centre of the umbrella (fig. 26). Histology of the Hydromedusa.—The histology described above for the polyp may be taken as the primitive type, from which that From Allman's Gymnoblastic Hydroids, by permission of the Council of the Ray Society. From Allman's Gymnoblastic Hydroids, by permission of the Council of the Ray Society. After E. T, Browne, from Prot. Zool. Soc. of London. After O. Maas, Craspedolen Medusen der Siboga-Expedition, by permission of E. S. Brill & Co. After O. Maas, Medusae, in Prinee of Monaco's series. Radial nerve. Tentaculocyst. Circular canal. Radiating canal. Ovary. Peronia or cartilaginous process ascending from the cartilaginous margin of the disk centripetally in the outer surface of the jelly-like disk; six of these are perradial, six interradial, corresponding to the twelve solid larval tentacles, resembling those of Cunina. k, Dilatation (stomach) of the manubrium. 1, Jelly of the disk. p, Manubrium. t, Tentacle (hollow and tertiary, i.e. preceded by six per-radial and six interradial solid larval tentacles). u, Cartilaginous margin of the disk covered by thread-cells. v, Velum. by which the animal progresses. The longitudinal system is discontinuous, and is subdivided into proximal, medial and distal portions. The proximal portion forms the retractor muscles of the manubrium, or proboscis, well. developed, for example, in Geryonia. The medial portion forms radiating tracts of fibres, the so-called " bell-muscles " running underneath, and parallel to, the radial canals; when greatly developed, as in Tiaridae, they form ridges, so-called mesenteries, projecting into the sub-umbra( cavity. The distal portions form the muscles of the tentacles. In contrast with the polyp, the longitudinal muscle-system is entirely ectodermal, there being no endodermal muscles in s..aspedote medusae. The nervous system of the medusa consists of sub-epithelial ganglion-cells, which form, in the first place, a diffuse plexus of nervous tissue, as in the polyp, but developed chiefly on the subumbral surface; and which are concentrated, in the second place, to form a definite central nervous system, never found in the polyp. In Hydromedusae the central nervous system forms two concentric nerve-rings at the margin of the umbrella, near the base of the velum. One, the " upper " forms part of the epidermal mosaic on the or ex-urnbral nerve- free surface of the body. (After Hertwig.) ring, is derived from the ectoderm on the ex-umbral side of the velum; it is the larger of the two rings, containing more numerous but smaller ganglion-cells, and innervates the tentacles. The other, the " lower or suburnbral nerve-ring, is derived from the ectoderm on the sub-umbral side of the velum.; it contains fewer but larger ganglion-cells and innervates the muscles of the velum (see diagram in article MEDUSAE). The two nerve-rings are connected by fibres passing from one to the other. The sensory cells are slender epithelial cells, often with a cilium or stiff protoplasmic process, and should perhaps be regarded as the only ectoderm-cells which retain the primitive ciliation of the larval ectoderm, otherwise lost in all Hydrozoa. The sense-cells form, in the first place, a diffuse system of scattered sensory cells, as in the polyp, developed chiefly on the manubrium, the tentacles and the margin of the umbrella, where they form a sensory ciliated epithelium covering the nerve-centres; in the second place, the sense-cells are concentrated to form definite sense-organs, situated always at the margin of the umbrella, hence often termed marginal bodies. The possession of definite sense-organs at once distinguishes the medusa from the polyp, in which they are never found. The sense-organs of medusae. are of two kinds—first, organs sensitive to light, usually termed ocelli (fig. 29) ; secondly, organs commonly termed otocysts, on account of their re= semblance to the auditory vesicles of higher animals, but serving for the sense of balance and orientation, and therefore given the special name of statocysts (fig. 30). The sense-organs may be tentaculocysts, i.e. modifications of a tentacle, as in Trachylinae, or developed from the margin of the umbrella, in no connexion with a tentacle (or, if so connected, not producing any modification in the tentacle), as in Leptolinae. In Hydromedusae the sense-organs are always exposed at the umbrellar margin (hence Gymnophthalmata), while rn Scyphomedusae they are covered over by flaps of the umbrellar margin (hence Steganophthalmata). The statocysts present in general the structure of either a knob or a closed vesicle, composed of (i) indifferent supporting epithelium; (2) sensory, so-called auditory epithelium of slender cells, each of the medusa differs only in greater elaboration and differentiation of the cell-elements, which are also more concentrated to form distinct tissues. The ectoderm furnishes the general epithelial covering of the body, and the muscular tissue, nervous system and sense-organs. The is-S- Fm. 26.-Carmarina (Geryonia) hastata, one of the Trachomedusae. (After Haeckel.) a, a', b, e, 4, lt, After O. Maas, Craspedolen Mediates der Siboga Expedition, by permission of E. S. Brill & Co. external epithelium is flat on the ex-umbral surface, more columnar on the sub-umbral surface, where it forms the muscular tissue of the sub-umbrella and the velum. The nematocysts of the ectoderm may be grouped to form batteries on the tentacles, umbrellar margin and oral lappets. In places the nematocysts may be crowded so thickly as to form a tough, supporting, "chondral " tissue, resembling cartilage, chiefly developed at the margin of the umbrella and forming streaks or bars supporting the tentacles (" Tentakelspangen, peronia) or the ten- taculocysts (' Gehorspangen, otoporpae). The muscular tissue of the Hydromedusae is entirely ectodermal. The muscle-fibres arise as processes from the bases of the epithelial cells; such cells may individually become sub-epithelial in position, as in the polyp; or, in places where muscular tissue rs greatly de- 'eloped, as in the velum or sub-umbrella, the entire muscular epithelium may be thrown into folds in order to increase its surface, so that a deeper sub-epithelial muscular layer becomes separated completely from a more superficial body epithelium. After O. Maas in Results of In its arrangement the muscular tissue theuseu•'mdlbal.ossf o " CSxpompeadratiitfon,veforms two systems: the one composed Jf Zoology, Cambridge, Mass., of striated fibres arranged' circularly, that U.S.A. is to say, concentrically round the central 142 HYDROMEDUSAE [ORGANIZATION bearing at its free upper end a stiff bristle and running out at its base into a nerve-fibre; (3) concrement-cells, which produce intercellular c.,c. concretions, so-called oto- liths. By means of vibrations or shocks transmitted through the –Sub water, or by displacements in the balance or position of the animal, the otoliths are caused to impinge against the bristles of the sensory cells, now on one side, now on the other, causing shocks or stimuli which are transmitted by the ~— _ basal nerve-fibre to the Sl.e'~'~r1>i+3. central nervous system, can oe Two stages in the de- velopment of the otocyst Modified after Linko, Travaux Soc. Imp. Nat., St. can be recognized, the Fetersbourg, sox. first that of an open pit In the Leptolinae the otocysts are seen in their first stage in Mitrocoina annae (fig. 31) and Tiaropsis (figs. 29, 30) as an open, pit at the base of the velum, on its subumbral side. The. pit has its opening turned towards the sub-umbral cavity, while its Z~ -• " st.c.~'~ base or fundus forms abulge, 'con more or less pronounced, on Modified after O. and R. Hertwig, Nerves- the ex-umbral side. of the system and Sinnesorgane der Medusas, by velum. At the fundus are permission of F. C. W. Vogel. placed the concrement-cells Fie. 3i.—Section of a Statocyst of with their conspicuous oto- Mitrocoma (tnnae. liths (con) and the inconspicu- sub, Sub-umbral ectoderm ous auditory cells, which are core Circular canal. connected with . the sub v. Velum. umbral nerve-ring. From st.c, Cavity of statocyst. the open condition arises con, Concrement-cell with otolith. the closed condition very simply by closing up of the aperture of the pit. We then find the typical otocyst of the Leptomedusae, a vesicle bulging on the ex-umbral side of the velum figs. 32, 33). The otocysts are' placed on the outer wall of the Sub Concrement - cell otolith. st.c, Cavity of statocyst. vesicle (the fundus of the original pit) or on its sides; their arrangement and number vary greatly and furnish useful characters for distinguishing genera. The sense-cells are innervated, as before, from the sub-umbral nerve-ring. The inner wall of the vesicle (region of closure) is frequently thickened to form a so-called " sense. cushion," apparently a ganglionic offshoot from the sub-umbral nerve-ring. In many Leptomedusae the otocysts are very small, in-conspicuous and em-bedded completely in the tissues; hence they may be easily overlooked in badly-preserved material, and perhaps are present in many cases where they end. have been said to have been wanting. In the Trachylinae the simplest condition of the otocyst is a freely projecting club, a so-called statorhabd (figs. 34, 35), representing a tentacle greatly reduced in size, covered with sensory ectodermal epithelium (eel.), and containing an endodermal core (end.), which is at first continuous with the endoderm of the ring-canal, but later becomes separated from it. In the endoderm large concretions are formed (con.). Other sensory cells with long cilia cover a sort of cushion (sec.) at the base of the club.; the club may be long and the cushion small, or the ter', ._. -": cushion large and the club small. The whole structure is innervated, like the tentacles, from the, ex-umbral nerve-ring. An ' advance towards the second stage is seen in such a form as Rhopalonema (fig. 36), where the ectoderm of the cushion rises up in a double fold to enclose the , club in a protective covering forming a cup or vesicle, at first open distally; finally the opening closes and the closed vesicle may sink inwards and be found:. far removed from After O. and R. Hertwig, Nervensystem and Sfnnesthe surface, as in Geryonia organ der Medusen, by permission of F. C. W. (fig. 37). Vogel. The ocelli are seen in FIG. 35.—Tentaculocyst of Conine latitheir simplest form as a ventris. pigmented patch of ecto- ect, Ectoderm. deem, which consists of n.c, Nerve-cushion. two kinds of cells—(1) end, Endodermal concrement-cells. pigment-cells, which are con, Otolith. ordinary indifferent cells of the epithelium containing pigment-granules, and (2) visual cells, slender sensory epithelial cells of the usual type, which may develop visual cones or rods at their free extremity. The ocelli occur usually either on the inner or outer sides of the tentacles; if on the inner side, the tentacle is turned upwards and carried over the ex-umbrella, so as to expose the ocellus to the light; if the FIG. 36.-Simple tentaculocyst of Rhopalo• ocellus be on the sterna velatum. The process carrying the otolith outer side of a or concretion hk, formed by endoderm cells, is tentacle, two enclosed by an upgrowth, forming the " vesicle," nerves ruit round which is not yet quite closed in at the top. the . base of the , (After Hertwig.) tentacle to it. In other cases ocelli may occur between tentacles, as in Tiara psis (fig. 29). The simple form of ocellus described in the foregoing paragraph may become folded into a pit or cup, the interior of which becomes filled with a clear gelatinous secretion forming a sort of vitreous ex, Ex-umbral ectoderm. sub, Sub-umbral ectoderm. roc, Circular canal. v, Velum. st.c, Cavity of statocyst. con, Concrement-cell with otolith. Modified after O. and R. Hertwig, Nerves-system and Sinnesorgane der Medusen, by permission of F. C. W. Vogel. ex, Ex-umbral ectoderm. sub, Sub-umbral ectoderm. v, Velum. six, Cavity of statocyst. con, Concrement-cell with otolith. V con, Modified after O. and R. Hertwig, A'ervensystem and Sinnesorgane der Medusen, by permission of F. C. W. Vogel. a Statocyst of Oclorchis. with After O. and R. Hertwig, Nervensystem and Sinnesorgane der Medusen, by permission of F. C. W. Vogel. body. The distal portion of the vitreous body may project from the cavity of the cup, forming a non-cellular lens as in Lizzia (fig. 28). Beyond this simple condition the visual organs of the Hydromedusae do not advance, and are far from reaching the wonderful development of the eyes of Scyphomedusae (Charybdaea). Besides the ordinary type of ocellus just described, there is found in one genus(Tiaropsis) a type of ocellus in which the visual elements are inverted, and have their cones turned away from the light, as in the human retina (fig. 30). In this case the pigment-cells are endodermal, forming a cup of pigment in which the visual cones are embedded. A similar ocellus is formed in Aurelia among the Sc phomedusac (q.v.). Other sense organs of Hydromedusae are the so-called sense-clubs or cordyli found in a few Leptomedusae, especially in those genera in which otocysts are incon- nr After O. and R. Hertwig, Nerve:system and Sinnesorgane lei Medusen, by permission of F. C. W. Vogel. st.c, Statocyst containing the minute tentaculocyst. nri, Ex-umbral nerve-ring. nrq, Sub-umbral nerve-ring. ex, Ex-umbral ectoderm. sub, Sub-umbral ectoderm. c.c, Circular canal. spicuous or absent v, Velum. (fig. 39). Each cordylus is a ten- tacle-like structure with an endodermal axis containing. an axial cavity which may be continuous with the ring-canal, or may be partially occluded. Externally the cordylus is covered by very flattened ectoderm, and bears no otoliths or sense-cells, but the base of the club rests upon the ex-umbral nerve-ring. Brooks regards these organs as sensory, serving for the sense of balance, and representing a primitive stage of the tentaculocysts of Trachylinae; Linko, on the other hand, finding no nerve-elements connected with them, regards them as digestive (?) in function. The sense-organs of the two fresh-water medusae Limnocodium and Limnocnida are peculiar and of rather doubtful nature (see E. T. Browne [10]). The endoderm of the medusa shows the same general types of structure as in the polyp, described above. We can distinguish (I) digestive endoderm, in the stomach, often with special glandular elements; (2) circu- latory endoderm, in the radial and ring- canals; (3) supporting endoderm in the axes of the tentacles and in the endoderm- lamella; the latter is primitively a double layer of cells, produced by concrescence of the ex-umbral and sub-umbra] layers of the coelenteron, but it is usually found as a single layer of flattened cells (fig. 40) ; in Geryonia, however, it remains double, and the centripetal canals arise by parting of the two layers; (4) excretory endoderm, lining pores at the margin of the umbrella, occurring in certain Leptomedusae as so- called " marginal tubercles," opening, on the one hand, into the ring-canal and, on the other hand, to the exterior by " marginal funnels," which debouch into the sub-umbral cavity above the velum. As has been de- scribed above, the endoderm may also con- tribute to the sense-organs, but such contributions are always of an accessory nature, for instance, concrement-cells in the otocysts, pigment in the ocelli, and never of sensory nature, sense-cells being in all cases ectodermal. The reproductive cells may be regarded as belonging primarily to neither ectoderm nor endoderm, though lodged in the ectoderm in all Hydromedusae. As described for the polyp, they are wandering cells capable of extensive migrations before reaching the particular spot at which they ripen. In the Hydromedusae they usually, if not invariably, ripen in the ectoderm, but in the neighbourhood of the main sources of nutriment, that is to say, not far from the stomach. Hence the gonads are found on the manubrium in Anthomedusae generally; on the base of the manubrium, or under the gastral pouches, or in both these situations (Octorchidae), or under the radial canals, in Trachomedusae; under the gastral pouches or radial canals, in Narcomedusae. When ripe, the germ-cells are dehisced directly to the exterior. Hydromedusae are of separate sexes, the only known exception being Amphogona apsteini, one of the Trachomedusae (Browne [9]). Moreover, all the medusae budded from a given hydroid colony are either male or female, so that even the non-sexual polyp must be considered to have a latent sex. (In Hydra, on the other hand, the individual is usually hermaphrodite.) The medusa always reproduces itself sexually, and in some cases non-sexually also. The non-sexual reproduction takes the form of fission, budding or sporogony, the details of which are described below. Buds may be produced from the manubrium, radial canals, ring-canal, or tentacle-bases, or from an aboral stolen (Narcomedusae). In all cases only medusa-buds are produced, never polyp-buds. The mesogloea of the medusa is largely developed and of great thickness in the umbrella. The sub-epithelial tissues, i.e. the nervous and muscular cells, are lodged in the mesogloea, but in Hydromedusae it never contains tissue-cells or mesogloeal corpuscles. (b) The Medusae as a Subordinate Individuality. —It has been shown above that polyps are budded only from polyps and that the medusae may be budded either from polyps or from medusae. in any case the daughter-individuals produced from the buds may be imagined ae remaining attached to the parent and forming a colony of individuals in organic connexion with one another, and thus three possible cases arise. The first case gives a colony entirely composed of polyps, as in many Hydroidea: The second case gives a colony partly composed of polyp-individuals, partly of medusa-individuals, a possibility also realized in many colonies of Hydroidea. The third case gives a colony entirely composed of medusa-individuals, a possibility perhaps realized in the Siphonophora, which will be discussed in dealing with this group. The first step towards the formation of a mixed hydroid colony is undoubtedly a hastening of the sexual maturity of the medusa-individual. Normally the medusae are liberated in quite an immature state; they swim away, feed, grow and become adult mature individuals. From the bionomical point of view, the medusa is to be considered as a means of spreading the species, supplementing the deficiencies of the sessile polyp. It may be, however, that in-creased reproductiveness becomes of greater importance to the species than wide diffu- sion; such a condition FIG. 4o.—Portions of Sections through will be brought about if the Disk of Medusae—the upper one of the medusae mature Lizzia, the lower of Aurelia. (After quickly and are either Hertwig.) set free in a mature el, Endoderm lamella. condition or remain in m, Muscular processes of the ectoderm-cells the shelter of the polyp- in cross section. colony, protected from d, E ctoderm. risks of a free life in the en, Endoderm lining the enteric cavity. open sea. In this way e Wandering endoderm cells of the the medusa sinks from ' gelatinous substance. an independent per- sonality to an organ of the polyp-colony, becoming a so-called medusoid gonophore, or bearer of the reproductive organs, and losing gradually all organs necessary for an independent existence, namely those of sense, locomotion %and nutrition. In some cases both free medusae and gonophores may be produced from the same hydroid colony. This is the case in Syncoryne mirabilis (Allman [1], p. 278) and in Campanularia volubilis; in the latter, free medusae are produced in summer, gonophores in winter (Duplessis [14]). Again in Pennaria, the male medusae are set free After W.!K. Brooks, Journal of Morphology, L, by permission of Ginn & Co. Laodice. c.c, Circular canal. v, Velum. t, Tentacle. c, Cordylus, composed of flattened ectoderm ec covering a large-celled endodermal axis en. in a state of maturity, and have ocelli; the female medusae remain attached and have no sense organs. The gonophores of different hydroids differ greatly in structure from one another, and form a series showing degeneration of the medusa-individual, which is gradually stripped, as it were, of its characteristic features of medusan organization and finally reduced to the simplest structure. A very early stage in the degeneration is well exemplified by the so-called " meconidium " of Gonothyraea (fig. 41, A). Here the medusoid, attached by the centre of its ex-umbral surface, has lost its velum and sub-umbral muscles, its sense organs and mouth, though still retaining rudimentary tentacles. The gonads (g) are produced on the manubrium, which has a hollow endodermal axis, termed the spadix (sp.), in open communication with the coenosarc of the polyp-colony and serving for the nutrition of the generative cells. A very similar condition is seen in Tubularia (fig. 41, B), where, however, the tentacles have quite disappeared, and the circular rim formed by the margin of the umbrella has nearly closed over the manubrium leaving only a small aperture through which the embryos emerge. The next step is illustrated by the female gonophores of Cladocoryne, where the radial and ring-canals Modified from Weisman, Entslehung der Sexualzelten bei den Hydromedusen. A, " Meconidium" of Gonothyraea. H, Withspadixbranched(Cordy- B, Type of Tubularia. lophora). C, Type of Garveia, &c. [&c. s.c, Sub-umbral cavity. D, Type of Plumularia, Agalma, t, Tentacles. E, Type of Coryne, Forskalia,&c. c.c, Circular canal. F, G, H, Sporosacs. g, Gonads. F, With simple spadix. sp, Spadix. G, With spadix prolonged c.l, Endoderm-lamella. (Eudendrium). ex, Ex-umbral ectoderm. ect, Ectotheca. have become obliterated by coalescence of their walls, so that the entire endoderm of the umbrella is in the condition of the endoderm-lamella. Next the opening of the umbrella closes up completely and disappears, so that the sub-umbral cavity forms a closed space surrounding the manubrium, on which the gonads are developed; such a condition is seen in the male gonophore of Cladocoryne and in Garveia (fig. 41, C), where, however, there is a further complication in the form of an adventitious envelope or ectotheca (ect.) split off from the gonophore as a protective covering, and not present in Cladocoryne. The sub-umbral cavity (s.c.) functions as a brood-space for the developing embryos, which are set free by rupture of the wall. It is evident that the outer envelope of the gonophore represents the ex-umbral ectoderm (ex.), and that the inner ectoderm lining the cavity represents the sub-umbral ectoderm of the free medusa. The next step is the gradual obliteration of the sub-umbral cavity (s.c.) by disappearance of which the sub-umbral ectoderm comes into contact with the ectoderm of the manubrium. Such a type is found in Plumularia and also in Agalma (fig. 41, D); centrally is seen the spadix (sp.), bearing the generative cells (g), and external to these (I) a layer of ectoderm representing the epithelium of the manubrium; (2) the layer of sub-umbral ectoderm; (3) the endoderm-lamella (e.l.) ; (4) the ex-umbral ectoderm (ex.) ; and (5) there may or may 'not be present also an ectotheca. Thus the gonads are covered over by at least four layers of epithelium, and since these are unnecessary, presenting merely obstacles to the dehiscence of the gonads, they gradually undergo reduction. The sub-umbral ectoderm and that covering the manubrium undergo concrescence to form a single layer (fig. 41, E), which finally disappears altogether, and the endoderm-lamella disappears. The gonophore is now reduced to its simplest condition, known as the sporosac (fig. 41, F, G, H), and consists of the spadix bearing the gonads covered by a single layer of ectoderm (ex.), with or without the addition of an ectotheca. It cannot be too strongly emphasized, however. that the sporosac should not be compared simply with the manubrium of the medusa, as is sometimes done. The endodermal spadix (sp) of the sporosac represents the endoderm of the manubrium; the ectodermal lining of the sporosac (ex.) represents the ex-umbral ectoderm of the medusa; and the intervening layers, together with the sub-umbral cavity, have disappeared. The spadix, as the organ of nutrition for the gonads, may be developed in various ways, being simple (fig. 1, F) or branched (fig. 41, H); in Eudendrium (fig. 4r, G) it curls round the single Large ovum. The hydroid Dicoryne is remarkable for the possession of gonophores, which are ciliate and become detached and swim away by means of their cilia. Each such sporosac has two long tentacle-like processes thickly ciliated. It has been maintained that the gonads of Hydra represent sporosacs or gonophores greatly reduced, with the last traces of medusoid structure completely obliterated. There is, however, no evidence whatever for this, the gonads of Hydra being purely ectodermal structures, while all medusoid gono- phores have an endodermal portion. Hydra is, moreover, bisexual, In contrast with what is known of hydroid colonies. In some Leptomedusae the gonads are formed on the radial canals and form protruding masses resembling sporosacs superficially, but not in structure. Allman, however, regarded this type of gonad as equivalent to a sporosac, and considered the medusa bearing them as a non-sexual organism, a " blastocheme ,,he termed it, producing by budding medusoid gonophores. As medusae are known to bud medusae from the radial canals there is nothing impossible in Allman's theory, but it cannot be said to have received satisfactory proof. Reproduction and Ontogeny of the Hydromedusae. Nearly every possible method of reproduction occurs amohgst the Hydromedusae. In classifying methods of generation it is usual to make use of the sexual or non-sexual nature of the reproduction as a primary difference, but a more scientific classification is afforded by the distinction between tissue-cells After Allman, Gymnoblastic Hydroids, by permission of the Council of the Ray Society. Ftc. 42.-Gonophores of Dicoryne conferta. A, A male gonophore still enclosed in its ecto- theca. [liberation. B and C, Two views of a female gonophore after t, Tentacles. ov, Ova, two carried on each female gonophore. sp, Testis. (histocytes) and germinal cells, actual or potential (archaeocytes), amongst the constittient cells of the animal body. In this way we may distinguish, first, vegetative reproduction, the result of discontinuous growth of the tissues and cell-layers of the body as a whole, leading to (r) fission, (2) autotomy, or (3) vegetative budding; secondly, germinal reproduction, the, result of the reproductive activity of the archaeocytes or germinal tissue. In germinal reproduction the proliferating cells may be undifferentiated, so-called primitive germ-cells, or they may be differentiated as sexual cells, male or female, i.e. spermatozoa and ova. If the germ-cells are undifferentiated, the offspring may arise from many cells or from a single cell; the first type is (4) germinal budding, the second is (5) sporogony. If the germ-cells are differentiated, the offspring arises by syngamy or sexual union of the ordinary type between an ovum and spermatozoon, so-called. fertilization of the ovum, or. by parthenogenesis, i.e. development of an ovum without fertilization. The only one of these possible modes of reproduction not known to occur in Hydromedusae is parthenogenesis. (i) True fission or longitudinal division of an individual into two equal and similar daughter-individuals is not common but occurs in Gastroblasta, where it has been described in detail by Arnold Lang ]30]. ' (2) Autotomy, sometimes termed transverse fission, is the name given to a process of unequal fission in which a portion of the body separates off with subsequent regeneration. In Tubularia by a process of decapitation the, hydranths may separate off and give rise to a separate individual, while the remainder of the body grows a new hydranth. Similarly in Schizoeladium portions of the hydrocaulus are cut off to form so-called " spores," which grow into new individuals (see Allman [1]). (3) Vegetative bud-ding is almost universal in the Hydromedusae. By budding is understood the formation of a new individual from a fresh growth of undifferentiated material. It is convenient to distinguish buds that give rise to polyps from those that form medusae. (a). The Polyp.—The buds that form polyps are very simple in mode of formation. Four stages may be distinguished; the first is a simple outgrowth of both layers, ectos.c. derma and endoderm, / ~o. containing a prolonga- ~s.tion of . the coelenteric F v cavity ; in the second ~t stage the tentacles Much modified from C. Chun, °Coelenterata," in grow out as secondary Bronn's Twrrcich. diverticula from the A, B, C, E, F, In ver- t, Tentacle. growth; in the third tical section. s.o, Sense organ. stage the mouth is I), Sketch of exter- v, Velum. formed as a perfora- nat view. s.c, Sub -umbra 1 non of the two layers ; st, Stomach. cavity. and, lastly, if the bud rn, Manubrium. n.s, Nervous system. is to be separated, it becomes nipped off from the parent polyp and begins a free existence. (b) The Medusae.—Two types of budding must be distinguished —the direct, so-called palingenetic type, and the indirect, so-called cocnogenetic type. The direct type of budding is rare, but is seen in Cunina and Millepora. In Cunina there arises, first, a simple outgrowth of both layers, as in a polyp-bud (fig. 43, A) ; in this the mouth is formed chstally as a perforation (B) ; next the sides of the tube so formed D A, B, C, D, F, Succes- sub -umbra l sive stages in ver- cavity. tical section. st, Stomach. E, Transverse sec- r.c. Radial canal. tion of a stage c.c, Circular canal. similar to D. e.l, Endoderm lamella. Gc, Entocodon. m, Manubrium. s.c, Cavity of ento- v, Velum. codon, forming t, Tentacle. the future bulge out laterally near the attachment to form the umbrella, while the distal undilated portion of the tube represents the manubrium (C); the umbrella now grows out into a number. of lobes or lappets, and the tentacles and tentaculocysts grow out, the former in a notch between two lappets, the latter on the apex of each lappet (D, E); finally, the velum arises as a growth of the ectoderm alone; the whole bud shapes itself, so to speak, and the little medusa is separated off by rupture of the thin stalk connecting it with the parent (F). The direct method of medusa-budding only differs from the polyp-bud by its greater complexity of parts and organs. The indirect mode of budding (figs. 44, 45) is the commonest method by which medusa-buds are formed. It is marked by the formation in the bud of a characteristic structure termed the entocodon (Knospenkern, Glockenkern). The first stage is a simple hollow outgrowth of both body-layers (fig. 44, A) ; at the tip of this is formed a thickening of the ectoderm, arising primitively as a hollow ingrowth (fig. 44, B), but more usually as a solid mass of ectoderm-cells (fig. 45, A). The ectodermal ingrowth is the entocodon (Gs.) ; it bulges into, and pushes down, the endoderm at the apex of the bud, and if solid it soon acquires a cavity (fig. 44, C, s.c.). The cavity of the entocodon increases continually in size, while: the endoderm pushes up at the sides of it to form a cup with hollow walls, enclosing but not quite surrounding the entocodon, which remains in contact at its outer side with the ectoderm covering the bud (fig. 44, D, v). The next changes that take place are chiefly In the endoderm-cup (fig. 44, D, E) ; the cavity A 8 C between the two walls of the cup FIG. 45.—Modifications of the method of becomes reduced budding shown in fig. 44, with solid Entoby concrescence to codon (Gc.) and formation of an ectotheca (ea.). form the radial canals (r.c.), ring-canal (c.c.), and endoderm-lamella (e1., fig. 44, E), and at the same time the base of the cup is thrust upwards to form the manubrium (m), converting the cavity of the entocodon into a
End of Article: HYDROMEDUSAE
HYDROMECHANICS (Gr. ubpops avuca)
HYDROMETER (Gr. u"Swp, water, and pErpov, a measure...

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