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LAMELLIBRANCHIA (Lat. lamella, a smal...

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Originally appearing in Volume V16, Page 121 of the 1911 Encyclopedia Britannica.
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LAMELLIBRANCHIA (Lat. lamella, a small or thin plate, and Gr. /3payxia, gills), the fourth of the five classes of animals constituting the phylum Mollusca (q.v.). The Lamellibranchia are mainly characterized by the rudimentary condition of the head, and the retention of the primitive bilateral symmetry, the latter feature being accentuated by the lateral compression of the body and the development of the shell as two bilaterally symmetrical plates or valves covering each one side of the animal. The foot is commonly a simple cylindrical or ploughshare-shaped organ, used for boring in sand and mud, and more rarely presents a crawling- disk similar to that of Gastropoda; in some forms it is aborted. The paired ctenidia are very greatly developed right and left of the elongated body, and form the most prominent organ of the group. Their function is chiefly not respiratory but nutritive, since it is by the currents produced by their ciliated surface that food-particles are brought to the feebly-developed mouth and buccal cavity. The Lamellibranchia present as a whole a somewhat uniform structure. The chief points in which they vary are—(1) in the structure of the ctenidia or branchial plates; (2) in the presence of one or of two chief muscles, the fibres of which run across the animal's body from one valve of the shell to the other (adductors) ; (3) in the greater or less elaboration of the posterior portion of the mantle-skirt so as to form a pair of tubes, by one of which water is introduced into the sub-pallial chamber, whilst by the other it is expelled; (4) in the perfect or deficient symmetry of the two valves of the shell and the connected soft parts, as compared with one another; (5) in the development of the foot as a disk-like crawling organ (Area, Nucula, Pectunculus, Trigonia, Lepton, Galeomma), as a simple plough-like or tongue-shaped organ (Unionidae, &c.), as a re-curved saltatory organ (Cardium, &c.), as a long burrowing cylinder (Solenidae, &c.), or its partial (Mytilacea) or even complete abortion (Ostraeacea). The essential Molluscan organs are, with these exceptions, uniformly well developed. The mantle-skirt is always long, and hides the rest of the animal from view, its dependent margins meeting in the middle line below the ventral surface when the animal is retracted; it is, as it were, slit in the median line before and behind so as to form two flaps, a right and a left; on these the right and the left calcareous valves of the shell are borne respectively, connected by an uncalcified part of the shell called the ligament. In many embryo Lamellibranchs a centro-dorsal primitive shell-gland or follicle has been detected. The mouth lies in the median line anteriorly, the anus in the median line posteriorly. Both ctenidia, right and left, are invariably present, the axis of each taking origin from the side of the body as in the schematic archi-Mollusc (see fig. 15). A pair of renal tubes opening right and left, rather far forward on the sides of the body, are always present. Each opens by its internal extremity into the pericardium. A pair of genital -apertures, connected by genital ducts with the paired gonads, are found right and left near the nephridial pores, except in a few cases where the genital duct joins that of the renal organ (Spondylus). The sexes are often, but not always, distinct. No accessory glands or copulatory organs are ever present in Lamellibranchs. The ctenidia often act as brood-pouches. A dorsal contractile heart, with symmetrical right and left auricles receiving aerated blood from the ctenidia and mantle- skirt, is present, being unequally developed only in those few forms which are inequivalve. d he typical pericardium is well developed. It, as in other Mollusca, is not a blood-space but develops from the coelom, and it communicates with the exterior by the pair of renal tubes. As in Cephalopoda (and possibly other Mollusca) water can be introduced through the nephridia into this space. The alimentary canal keeps very nearly to the median vertical plane whilst exhibiting a number of flexures and loopings in this plane. A pair of large glandular outgrowths, the so-called " liver " or great digestive gland, exists as in other Molluscs. A pair of pedal otocysts, and a pair of osphradia at the base of the gills, appear to be always present. A typical nervous system is present (fig. 19), consisting of a cerebro-pleural ganglion-pair, united by connectives to a pedal ganglion-pair and a visceral ganglion-pair (parietosplanchnic). A pyloric caecum connected with the stomach is commonly found, containing a tough flexible cylinder of transparent cartilaginous appearance, called the " crystalline style " (Mactra). In many Lamellibranchs a gland is found on the hinder surface of the foot in the mid line, which secretes a substance which sets into the form of threads—the so-called " byssus "—by means of which the animal can fix itself. Sometimes this gland is found in the young and not in the adult (Anodonta, Unio, Cyclas). In some Lamellibranchs (Peden, Spondylus, Pholas, Afactra, Tellina, Pectunculus, Galeomma, &c.), although cephalic eyes are generally absent, special eyes are developed on the free margin of the mantle-skirt, apparently by the modification of tentacles commonly found there. There are no pores in the foot at elsewhere in Lamellibranchia by which water can pass into and out of the vascular system, as formerly asserted. The Lamellibranchia live chiefly in the sea, some in fresh waters. A very few have the power of swimming by opening and shutting the valves of the shell (Pecten, Lima); most can crawl slowly or burrow rapidly; others are, when adult, permanently fixed to stones or rocks either by the shell or the byssus. In development some Lamellibranchia pass through a free-swimming trochosphere stage with preoral ciliated band; other fresh- (1) ' A a e m l C :1 ~' rr c a•a9 Letters in all the figures as follows: a, Centro-dorsal area. of the inner lamella of the b, Margin of the left mantle- inner gill-plate and the side flap. of the foot, through which c, Margin of the right mantle- the probe g passes into the flap. upper division of the sub- d, Excurrent siphonal notch of pallial space. the mantle margin. aa, Line of concrescence of the e, Incurrent siphonal notch of inner lamella of the right the mantle margin. inner gill-plate with the f, Foot. inner lamella of the left g, Probe passed into the inner gill-plate. superior division of the sub- ab, ac, ad, Three pit-like depress pallial chamber through the sions in the median line excurrent siphonal notch, of the foot supposed by and issuing by the side of some writers to be pores ad- the foot into the inferior mitting water into the division of the sub-pallial vascular system. chamber. ae, Left shell valve. h, Anterior (pallial) adductor af, Space occupied by liver. muscle of the shells. ag, Space occupied by gonad. Anterior retractor muscle of ah, Muscular substance of the the foot. foot. k, Protractor muscle of the foot. ai, Duct of the liver on the wall 1, Posterior (pedal) adductor of the stomach, muscle of the shells. ak, Stomach. m, Posterior retractor muscle of al, Rectum traversing the yen- the foot. tricle of the heart. n, Anterior labial tentacle, am, Pericardium. o, Posterior labial tentacle. an, Glandular portion of the left p, Base-line of origin of the re- nephridium. fleeted mantle-flap from the ap, Ventricle of the heart. side of the body. eq, Aperture by which the left q, Left external gill-plate. auricle joins the ventricle. r, Left internal gill-plate. ar, Non-glandular portion of the re, Inner lamella of the right left nephridium, inner gill-plate. as, Anus. rg, Right outer gill-plate. at, Pore leading from the peni- s, Line of concrescence of the cardium into the glandular outer lamella of the left sac of the left nephridium. outer gill-plate with the left an, Pore leading from the gland- mantle-flap. ular into the non-glandular t, Pallial tentacles. portion of the left neph- u; The thickened muscular radium. pallial margin which ad- av, Internal pore leading from heres to the shell and forms the non-glandular portion the pallial line of the left of the left nephridium to the side. external pore x. v, That of the right side. aw, Left cerebro-pleuro-visceral w, The mouth. ganglion. x, Aperture of the left organ ax, Left pedal ganglion. of Bojanus (nephridium) ay, Left otocyst. exposed by cutting the az, Left olfactory ganglion attachment of the inner (parieto-splanchnic). lamella of the inner gill- bb, Floor of the pericardium plate. separating that space from y, Aperture of the genital duct. the non-glandular portion of z, Fissure between the free edge the nephridia. water forms which carry the young in brood-pouches formed by the ctenidia have suppressed this larval phase. As an example of the organization of a Lamellibranch, we shall review the structure of the common pond-mussel or swan mussel (Anodonta cygnea), comparing it with other Lamellibranchia. The swan-mussel has superficially a perfectly developed bilateral . symmetry. The left side of the animal is seen as when removed from its shell in fig. i (I). The valves of the shell have been removed by severing their adhesions to the muscular areae h, i, k, 1, m, u. The free edge of the left half of the mantle-skirt b is represented as a little contracted in order to show the exactly similar free edge of the right half of the mantle-skirt c. These edges are not attached to, although they touch, one another; each flap (right or left) can be freely thrown back in the way carried out in fig. r (3) for that of the left side. This is not always the case with Lamellibranchs; there is in the group a tendency for the corresponding edges of the mantle-skirt to fuse together by concrescence, and so to form a more or less completely closed bag, as in the Scaphopoda (Dentalium). In this way the notches d, e of the hinder part of the mantle-skirt of Anodonta are in the siphonate forms converted into two separate holes, the edges of the mantle being elsewhere fused together along this hinder margin. Further than this, the part of the mantle-skirt bounding the two holes is frequently drawn out so as to form a pair of tubes which project from the shell (figs. 8, 29). In such Lamellibranchs as the oysters, scallops and many others which have the edges of the mantle-skirt quite free, there are numerous tentacles upon those edges. In Anodonta these pallial tentacles are confined to a small area surrounding the inferior siphonal notch (fig. I [31, t). When the edges of the mantle ventral to the inhalant orifice are united, an anterior aperture is left for the protrusion of the foot, and thus there are three pallial apertures altogether, and species in this condition are called " Tripora." This is the usual condition in the Eulamellibranchia and Septibranchia. When the pedal aperture is small and far forward there may be a fourth aperture in the region of the fusion behind the pedal aperture. This occurs in Solen, and such forms are called " Quadrifora." The centro-dorsal point a of the animal of Anodonta (fig. I [ii) is called the umbonal area; the great anterior muscular surface his that of the anterior adductor muscle, the posterior similar surface i is that of the posterior adductor muscle; the long line of attachment u is the simple " pallial muscle,"—a thickened ridge which is seen to run parallel to the margin of the mantle-skirt in this Lamellibranch. In siphonate forms the pallial muscle is not simple, but is in- dented posteriorly by a sinus formed by the muscles which retract the siphons. It is the approximate equality in the size of the anterior and posterior adductor muscles which led to the name Isomya for the group to which Anodonta belongs. The hinder adductor muscle is always large in Lamellibranchs, but the anterior adductor may be very small (Heteromya), or absent altogether (Monomya). The anterior adductor ductor is ventral and anterior to the anus. The former classification based on these differences in the adductor muscles is now abandoned, having proved to be an un- natural one. A single family may include isomyarian, anisomyarian and monomyarian forms, and the latter in development pass through stages in which they resemble the first two. In fact all Lamellibranchs begin with a condition in which there is only one adductor, and that not the posterior but the anterior. This is called the protomono- myarian stage. Then the posterior adductor develops, and becomes equal to the anterior, and finally in some cases the anterior becomes smaller or disappears. The single adductor muscle of the Monomya is separated by a difference of fibre into two portions, but neither of these can be regarded as possibly representing the anterior adductor of the other Lamellibranchs. One of these portions is more liga- mentous and serves to keep the two shells con- stantly attached ...:s to one another, lauzzle whilst the more fleshy portion serves to close the shell rapidly when it has been gaping. In removing the valves of the shell from an Anodonta, it is necessary not only to cut through the muscular attachments it of the body-wall ralcrinferior Loril to the shell but to the Outer Face. elastic ligament, or spring resem- bling india-rubber, joining the two shells about the umbonal area. The shell of Anodonta does not present these parts in the most strongly marked condition, and accordingly our figures (figs. 2, 3, 4) represent the valves of the sinupalliate genus Cytherea. The corresponding parts are recognizable in Anodonta. Referring to the figures (2, 3) for an explanation of terms applicable to the parts of the valve and the markings on its inner surface -corresponding to the muscular areas already noted on the surface of the animal's body—we must specially note here the position of that denticulated thickening of the dorsal margin of the valve which is called the hinge (fig. 4). By this hinge one valve is closely fitted to the other. Below this hinge each shell becomes concave, above it each shell rises a little to form the umbo, and it is into this ridge-like upgrowth of each valve that the elastic ligament or spring is fixed (fig: 4). As shown in the diagram (fig. 5) representing a transverse section of the two valves of a Lamellibranch, the two shells form a double lever, of which the toothed-hinge is the fulcrum. The adductor muscles placed in the concavity of the shells act upon the long arms of the lever at a mechanical advantage; their contraction keeps the shells shut, and stretches the ligament or spring If. On the other hand, the ligament h acts upon the short arm formed by the umbonal ridge of the shells; whenever the adductors relax, the elastic substance of the ligament contracts, and the shells gape. It is on this account that the valves of a dead Lamellibranch always gape; the elastic ligament is no longer counteracted by the effort of the adductors. The state of closure of the valves of the shell is not, therefore, one of rest; when it is at rest—that is, when there is no muscular effort—the valves of a Lamellibranch are slightly gaping, and are closed by the action of the adductors when the animal is disturbed. The ligament is simple in Anodonta; in many Lamellibranchs it is separated into two layers, an outer and an inner (thicker; and denser). That the condition of gaping of the shell-valves is essential to the life of the Lamellibranch. appears from the fact that food to nourish it, water to aerate its FIG. 4.—Left Valve of the same Shell blood, and spermatozoa from the Inner Face. (Figs. 2, 3, 4 from to fertilize its eggs, are Owen.) all introduced into this gaping chamber by currents of water, set going by the highly-developed ctenidia. The current of water enters into the sub-pallial space at the spot marked e in fig. I (I), and, after passing as far for-ward as the mouth w in fig. I (5), takes an outward course and leaves the sub-pallial space by the upper notch d. These notches are known in Anodonta as the afferent and efferent siphonal notches respectively, and correspond to the long tube-like afferent inferior and efferent superior " siphons " formed by the mantle in many other Lamellibranchs (fig. 8). Whilst the valves of the shell are equal in Anodonta we find in many Lamellibranchs (Ostraea, Chama, Corbula, &c.) one valve larger, and the other smaller and sometimes flat, whilst the larger shell may be fixed to rock or to stones (Ostraea, &c.). A further variation consists in the development of additional shelly plates upon the dorsal line between the two large valves (Pholadidae). In Pholas dactylus we find a pair of umbonal plates, a dors-umbonal plate and a dorsal plate. It is to be remembered that the whole co of the cuticular hard product produced on the dorsal surface and on the mantle-flaps is to be regarded as the " shell," of which a median band-like area, the ligament, usually remains uncalcified, so as to result in the production of two valves united by the elastic ligament. But the shelly substance does not always in boring forms adhere to this form after its first growth. In Aspergillum the whole of the tubular mantle area secretes a continuous shelly tube, although in the young condition two valves were present. These are seen (fig. 7) set in the firm substance of the adult tubular shell, which has even re-placed the ligament, so that the tube is complete. In Teredo a similar tube is formed as the animal elongates (boring in wood), the original shell-valves not adhering to it but remaining movable and provided with a special muscular apparatus in place of a ligament. In the shell of Lamellibranchs three distinct layers can be distinguished : an external chitinous, non-calcified layer, the S py dr_ saponot6o,eT f lever; e, f, the long periostracum; a middle layer composed of arms of the lever; calcareous prisms perpendicular to the surface, g, the hinge; h, the the prismatic layer; and an internal layer ligament; i, the ad-composed of laminae parallel to the surface, ductor muscle. the nacreous layer. The last is secreted by the whole surface of the mantle except the border, and additions to its thickness continue to be made through life. The periostracum is produced by the extreme edge of the mantle border, the prismatic layer by the part of the border within the edge. These two layers, therefore, when once formed cannot increase in thickness; as the mantle grows in extent its border passes beyond the formed parts of the two outer layers, and the latter are covered internally by a deposit of nacreous matter. Special deposits of the nacreous matter around foreign bodies form pearls, the foreign nucleus being usually of parasitic origin (see PEARL). Let us now examine the organs which lie beneath the mantle-skirt of Anodonta, and are bathed by the current of water which circulates through it. This can be done by lifting up and throwing back the left half of the mantle-skirt as is represented in fig. i (3). We thus expose the plough-like foot (f), the two left labial tentacles, and the two left gill-plates or left ctenidium. In fig. I (5), one of the labial tentacles n is also thrown back to show the mouth w, and the two left gill-plates are reflected to show the gill-plates of the right side (rr, rq) pro- jecting behind the foot, the inner or median plate of each side being united by concrescence to its fellow of the opposite side along a continuous line (aa). The left inner gill-plate is also snipped to show the subjacent orifices of the left renal organ x, and of the genital gland (testis or ovary) y. The foot thus exposed in Anodonta is a simple muscular tongue-like organ. It can be pro- truded between the flaps of the mantle (fig. i [i] [2]) so as to issue from the shell, and by its action the Anodonta can slowly crawl or burrow in soft mud or sand. Other Lamelli- branchs may have a larger foot relatively than has Anodonta. In Arca it has a sole-like surface. In Arca too and many others it carries a byssus-forming gland and a byssus- cementing gland. In the cockles, in Cardium and in Trigonia, it is capable of a sudden stroke, which causes the animal to jump when out of the water, in the latter genus to a height of four feet. In Mytilus the foot is reduced to little more than a tubercle carrying the apertures of these glands. In the oyster it is absent altogether. The labial tentacles or palps of Anodonta (n, o in fig. i [31, [51) are highly vascular flat processes richly supplied with nerves. The left anterior tentacle (seen in the figure) is joined at its base in front of the mouth (w) to the right anterior tentacle, and similarly the left (o) and right posterior tentacles are joined behind the mouth. Those of Arca (i, k in fig. 9) show this relation to the mouth (a). These organs are characteristic of all Lamellibranchs; they do not vary except in size, being sometimes drawn out to streamer-like dimensions. Their appearance and position suggest that they are in some way related morphologically to the gill-plates, the anterior labial tentacle being a continuation of the outer gill :plate, and the posterior a continuation of the inner gill-plate. There is no embryolggical evidence to support this suggested connexion, and, as will appear immediately, the history of the gill-plates in various forms of Lamellibranchs does not directly favour it. The palps are really derived from part of the velar area of the larva. The gill-plates have a structure very different from that of the labial tentacles, and one which in Anodonta is singularly complicated as compared with the condition presented by these organs in some other Lamellibranchs, and with what must have been their originalcondition in the ancestors of the whole series of living Lamellibranchia. The phenomenon of " concrescence " which we have already had to note as showing itself so importantly in regard to the free edges of the mantle-skirt and the formation of the siphons, is what, above all things, has complicated the structure of the Lamellibranch ctenidium. Our present knowledge of the interesting series of modifications through which the Lamellibranch gill-plates have developed to their most complicated form is due to R. H. Peck, K. Mitsukuri and W. G. Ridewood. The Molluscan ctenidium is typically a plume- like structure, consisting of a vascular axis, on each side' of which is set a row of numerous lamelliform or filamentous processes. These processes are hollow, and receive the venous blood from, and return it again aerated into, the hollow axis, in which an afferent and an efferent blood-vessel may be differentiated. In the genus Nucula (fig. io) we have an example of a Lamellibranch retaining this plume-like form of gill. In the Arcacea (e.g. Arca and Pectunculus) the lateral processes which are set on the axis of the ctenidium are not lamellae, but are slightly flattened, very long tubes or hollow filaments. These filaments are so fine and are set so closely together that they appear to form a continuous membrane FIG. 9.—View from the ventral until examined with a lens. (pedal) aspect of the animal of The microscope shows that the Arca noae, the mantle-flap and neighbouring filaments are held gill-filaments having been cut together by patches of cilia, away. (Lankester.) called " ciliated junctions," a, Mouth. which interlock with one another b, Anus. just as two brushes may be c, Free spirally turned extremity made to do. In fig. II, A a of the gill-axis or ctenidial portion of four filaments of a axis of the right side. ctenidium of the sea-mussel d, Do. of the left side. (Mytilus) is represented, having e, f, Anterior portions of these axes precisely the same structure as fused by concrescence to the those of Arca. The filaments wall of the body. of the gill (ctenidium) of Mytilus g, Anterior adductor muscle. and Arca thus form two closely h, Posterior adductor. set rows which depend from the i, Anterior labial tentacle. axis of the gill like two parallel k, Posterior labial tentacle. plates. Further, their structure 1, Base line of the foot. is profoundly modified by the m, Sole of the foot. curious condition of the free n, Callosity. ends of the depending filaments. These are actually reflected at a sharp angle doubled on themselves in fact—and thus form an additional row of filaments (see fig. 11 B). Consequently, each primitive filament has a descending and an ascend- ing ramus, and instead of each row forming a simple plate, the plate is double, consisting of a descending and an ascending lamella. As the axis of the ctenidium lies by the side of the body, and is very frequently connate with the body, as so often happens in G9.stropods also, we find it convenient to speak of the two plate-like structures formed on each ctenidial axis as the outer and the inner gill-plate; each of these is composed of two lamellae, an outer (the reflected) and an adaxial in the case of the outer gill- plate, and an adaxial and an inner (the reflected) in the case of the inner gill-plate. This is the condition seen in Arca and Mytilus, the so-called plates dividing upon the slightest touch into their constituent filaments, which are but loosely conjoined by their " ciliated junctions." Complications follow upon this in other forms. Even in Mytilus and Area a connexion is here and there formed between the ascending and descending rami of a filament by hollow extensible outgrowths called " interlamellar junctions (il. j in B, fig. I I). Nevertheless the filament is a complete tube formed of chitinous substance and I clothed externally by ciliated epithelium, internally by endothelium and lacunar tissue—a form of connective tissue—as shown in fig. 11, C Now let us supposeas happens in the genus Dreissensid—a genus not far removed from Mytilus—that the ciliated inter-filamentar junctions (fig. 12) give place to solid permanent inter-filamentar junctions, so that the filaments are converted, as it were, into a trellis-work. Then let us suppose that the inter-lamellar junctions already noted in Mytilus become very numerous, large and irregular; by them the two trellis-works of filaments would be united so as to leave only a sponge-like set of spaces between them. Within the trabeculae of the sponge-work blood circulates, and between the trabeculae the water passes, having entered by the apertures left in the trellis-work formed by the united gill-filaments (fig. 14). The larger the intralamellar spongy growth becomes, the more do the original gill-filaments lose the character of blood-holding tubes, and tend to become dense elastic rods for the simple purpose of supporting the spongy growth. This is seen both in the section of Dreissensie gill (fig. 12) and in those of Anodonta (fig. 13, A,B,C). In the drawing of Dreissensia the individual filaments f,f,f are cut across in one lamella at the A tfda tf See also fig. 2. A. Section across the axis of a the left valve of the shell ctenidium with a pair of and the left half of the plates — flattened and shortened filaments—attached. i j,k,g Are placed on or near the membrane which attaches the axis of the ctenidium to the side of the body. a,b, Free extremities of the plates (filaments). d, Mid-line of the inferior border. e, Surface of the plate. t, Its upper border. h, Chitinous lining of the plate. r, Dilated blood-space. u, Fibrous tract. o, Upper blood-vessel of the axis. n, Lower blood-vessel of the axis. s, Chitinous framework of the axis. c Canal in the same. A, B, Line along which the cross-section C of the plate is taken. Animal of a male Nucula proxima, Say, as seen when horizon of an inter-filamentar junction, in the other (lower in the figure) at a point where they are free. The chitinous substance ch is observed to be greatly thickened as compared with what it is in fig. II, C, tending in fact to obliterate altogether the lumen of the filament. And in Anodonta (fig. 13, C) this obliteration is effected. In Anodonta, besides being thickened, the skeletal substance of the filament develops a specially dense, rod-like body on each side of each filament. Although the structure) of the etenidium is thus highly complicated in Anodonta, it is yet more so in some of the siphonate genera of Lamellibranchs. The filaments take on a secondary grouping, the surface of the lamella being thrown into a series of half-cylindrical ridges, each consisting of ten or twenty filaments; a filament of much greater strength and thickness than the others may be placed between each pair of groups. In Anodonta, as in many other Lamellibranchs, the ova and hatched embryos are carried for a time in the ctenidia or gill apparatus, and in this particular case the space between the two lamellae of the outer gill-plate is that which serves to receive the ova (fig. 13, A). The young are nourished by a substance formed by the cells which cover the spongy inter-lamellar outgrowths. Other points in the modification of the typical ctenidium must be noted in order to understand the ctenidium of Anodonta. The axis of each ctenidium, right and left, starts from a point well forward ment taken so as to cut neither a ciliated junction nor an inter-lamellar junction. i.e., Frontal epithelium; l.f.e'., l.f.e"., the two rows of latero-frontal epithelial cells with long cilia; ch, chitinous tubular lining of the filament; lac., blood lacuna traversed by a few processes of connective tissue cells; b.c., blood-corpuscle. near the labial tentacles, but it is at first only a ridge, and does not project as a free cylindrical axis until the back part of the foot is reached. This is difficult to see in Anodonta, but if the mantle-skirt be entirely cleared away, and if the dependent lamellae which spring from the ctenidial axis be carefully cropped so as to leave the +axis itself intact, we obtain the form shown in fig. 15, where g and h are respectively the left and the right ctenidial axes projecting freely beyond the body. In Arca this can be seen with far less trouble, for the filaments are more easily removed than are the consolidated lamellae formed by the filaments of Anodonta, and in Arca the free axes of the ctenidia are large and firm in texture (fig. 9, c,d). If we were to make a vertical section across the long axis of a Lamellibranch which had the axis of its ctenidium free from its origin onwards, we should find such relations as are shown in the diagram fig. 16, A. The gill axis d is seen lying in the sub-pallial chamber between the foot b and the mantle c. From it depend the gill-filaments or lamellae—formed by united filaments—drawn as black lines f. On the left side these lamellae are represented as having only a small reflected growth, on the right side the reflected ramus or lamella is complete (fr and er). The actual condition in Anodonta at the region where the gills begin anteriorly is shown in fig. 16, B. The axis of the ctenidium is seen to be adherent to, or fused by concrescence with, the body-wall, and moreover on each side the outer lamella of the outer gill-plate is fused to the mantle, whilst the inner lamella of the inner gill-plate is fused to the foot. If we take another section nearer the hinder margin of the foot, we get the arrangement A fief lac. ?se. Apex mantle-skirt are removed. a,a, Anterior adductor muscle. p.a, Posterior adductor muscle. v.m, Visceral mass. f, Foot. g, Gill. 1, Labial Tentacle. l.a, Filamentous appendage of the labial tentacle. lb, Hood-like appendage of the labial tentacle. Membrane suspending the gill and attached to the body along the line x, y, z, m, B. w. Posterior end of the gill (ctenidium). Section across one of the gill-plates (A, B, in A) comparable with fig. 11 C. Outer border. Axial border. Latero-frontal epithelium. Epithelium of general surface. Dilated blood-space. Chitinous lining (compare A). p, C. A,Part of four filaments seen from the outer face in order to show the ciliated junctions c.j. B,Diagram of the posterior face of a single complete filament with descending ramus and ascending ramus ending in a hook-like process;ep.,ep.,the ciliated junctions; inter-lamellar junction. C,Transverse section of a fila- Dreissensia polymorpha. (After R. H. Peck.) f, Constituent gill-filaments. bc, Blood-corpuscles. Fibroussub-epidermic tissue. fe, Frontal epithelium. ch, Chitonous substance of the lfe', lfe",Tworowsof latero-frontal filaments. epithelial cells with long nch, Cells related to the chitonous cilia. substance. lrf, Fibrous, possibly muscular, lac, Lacunar tissue. i substance of the inter- pig, Pigment-cells. filamentar junctions. al A ch c lac o.l (After R. H. Peck.) f, Constituent filaments. lac, Lacunar tissue. ch, Chitonous substance of the filament. chr, Chitonous rod embedded in the softer substance ch.' shown diagrammatically in fig. 16, C, and more correctly in fig. 17. In this region the inner lamellae of the inner gill-plates are no longer lamellar junction. The series of oval holes on the back of the lamella are the water-pores which open between the filaments in irregular rows separated horizontally by the transverse interfilmentar junctions. affixed to the foot Passing still farther back behind the foot, we find in Anodonta the condition shown in the section D, fig. 16. The axes i are now free; the outer lamellae of the outer gill-plates (er) still adhere by concrescence to the mantle-skirt, whilst the inner lamellae of the inner gill-plates meet one another and fuse by concrescence at g. In the lateral view of the animal with reflected mantle-skirt and gill-plates, d the line of concrescence of the inner lamellae of the inner gill-plates is readily seen ; it is marked as in fig. 1 (5). In the same figure the free part of the inner lamella of the inner FIG. 15.—Diagram of a view from gill-plate resting on the foot the left side of the animal of Anodonta is marked z, whilst the at- cygnaea, from which the mantle-skirt, t a c h e d p a r;t—the most the labial tentacles and the gill-filaanterior—has been snipped ments have been entirely removed so with scissors so as to show as to show the relations of the axis the genital and nephridial of the gill-plumes or ctenidia g, h. apertures x and y. The con- (Original.) crescence, then, of the free a, Centro-dorsal area. edge of the reflected lamellae b, Anterior adductor muscle. of the gill-plates of Anodon c, Posterior adductor muscle. is very extensive. It is im- d, mouth. portant, because such a e, Anus. concrescence is by no means f, Foot. universal, and does not g, Free portion of the axis of left occur, for example, in Mytilus or in Arca; further, because when its occurrence is once appreciated, the reduction of the gill-plates of Anodonta to the plume-type of the simplest ctenidium presents no difficulty; and, lastly, it has importance in reference to its physiological significance. The mechanical result of the concrescence of the outer lamellae to the mantle-flap, and of the inner lamellae to one another as shown in section D, fig. 16, is that the sub-pallial space is divided into two spaces by a horizontal septum: The upper space (i) communicates with the outer world f try . ,eImm~ f A, Outer gill-plate. B, Inner gill-plate. C, A portion of B more highly o.l, Outer lamella. [magnified. Inner lamella. v, Blood-vessel. Diagram of a block cut from the outer lamella of the outer gill-plate and seen from the inter-lamellar surface. f, Constituent filaments; trf, fibrous tissue of the transverse inter-filamentar junctions; v, blood-vessel ilj, Inter- ctenidium. h, Axis of right ctenidium. k, Portion of the axis of the left ctenidium which is fused with the base of the foot, the two dotted lines indicating the origins of the two rows of gill-filaments. m, Line of origin of the anterior labial tentacle. n, Nephridial aperture. o, Genital aperture. r, Line of origin of the posterior labial tentacle. by the excurrent or superior siphonal notch of the mantle (fig. I, d); the lower space communicates by the lower siphonal a fr f- ft f A, Shows two conditions with er, Reflected lamella of outer gill- f ree gill-axis. plate. B, Condition at foremost region f, Adaxial lamella of inner gill-in A nodonta. [donta. plate. C, Hind region of foot in Ano- fr, Reflected lamella of inner D, Region. altogether posterior to gill-plate. the foot in Anodonta. g, Line of concrescence of the a, Visceral mass. reflected lamellae of the two b, Foot. inner gill-plates. c, Mantle flap. h, Rectum. d, Axis of gill or ctenidium. i, Supra-branchial space of the e, Adaxial lamella of outer gill- sub-pallial chamber. plate. notch (e in fig. 1). The only communication between the two spaces, excepting through the trellis-work of the gill-plates, is by the slit (z in fig. i (5)) left by the non-concrescence of a part of the inner lamella of the inner gill-plate with the foot. A probe (g) is introduced through this slit-like passage, and it is seen to pass out by the excurrent siphonal notch. It is through this passage, or in-directly through the pores of the gill-plates, that the water introduced into the lower subpallial space must pass on its way to the excurrent siphonal notch. Such a subdivision of the pallial chamber, and direction of the currents set up within it do not exist in a number of Lamellibranchs which have the gill-lamellae comparatively free (Mytilus, Arca, Trigonia, &c.), and it is in these forms that m, Mantle-flap. lamellah. br, Outer, b'r', inner gill-plate—each In the 9th edition of this composed of two lamellae. Encyclopaedia Professor (Sir) f, Foot. E. R. Lankester suggested that Ventricle of the heart. these differences of gill-struc- Auricle. turewould furnish characters a, p,p', Pericardial cavity. of classificatory value, and i, Intestine. this suggestion has been followed out by Dr Paul Pelseneer in the classification now generally adopted. The alimentary canal of Anodonta is shown in fig. 1 (4). The mouth is placed between the anterior adductor and the foot; the anus opens on a median papilla overlying the posterior adductor, and discharges into the superior pallial chamber along which theexcurrent stream passes. The coil of the intestine in Anodonta is similar to that of other Lamellibranchs. The rectum traverses the pericardium, and has the ventricle of the heart wrapped, as it were, around it. This is not an unusual arrangement in Lamellibranchs, and a similar disposition occurs in some Gastropoda (Haliotis). A pair of ducts (ai) lead from the first enlargement of the alimentary tract called stomach into a pair of large digestive glands, the so-called liver, the branches of which are closely packed in this region (cif). The food of the Anodonta, as of other Lamellibranchs, consists of microscopic animal and vegetable organisms, brought to the mouth by the stream which sets into the sub-pallial chamber at the lower siphonal notch (e in fig. I). Probably a straining of water from solid particles is effected by the lattice-work of the ctenidia or gill-plates. The heart of Anodonta consists of a median ventricle embracing the rectum (fig. i8, A), and giving off an anterior and a posterior artery, A, Pericardium opened dorsally so as to expose the heart and the floor of the pericardial chamber d. B, Heart removed and floor of the pericardium cut away on the left side so as to open the non-glandular sac of the nephridium, exposing the glandular sac b, which is also cut into so as to show the probe f. C, Ideal pericardium and nephridium viewed laterally. D, Lateral view showing the actual relation of the glandular and non-glandular sacs of the nephridium. The arrows indicate the course of fluid from the pericardium out-wards. and of two auricles which open into the ventricle by orifices protected by valves. The blood is colourless, and has colourless amoeboid corpuscles floating in it. In Ceratisolen legumen, various species of Arca and a few other species the blood is crimson, owing to the presence of corpuscles impregnated with haemoglobin. In Anodonta the blood is driven by the ventricle through the arteries into vessel-like spaces, which soon become irregular lacunae surrounding the viscera, but in parts—e.g. the labial tentacles and walls of the gut—very fine vessels with endothelial cell-lining are found. The blood makes its way by large veins to a venous sinus which lies in the middle line below the heart, having the paired renal organs (nephridia) placed between it and that organ. Hence it passes through the vessels of the glandular walls of the nephridia right and left into the gill-lamellae, whence it returns through many openings into the widely-stretched auricles. In the filaments of the gill of Protobranchia and many Filibranchia the tubular cavity is divided by a more or less complete fibrous septum into two channels, for an afferent and efferent blood-current. The ventricle and auricles of Anodonta. lie in a pericardium which is clothed with a pavement endothelium (d, fig. 18). 9 k 9 D h a, Ventricle of the heart. b, Auricle. bb, Cut remnant of the auricle. c, Dorsal wall of the pericardium cut and reflected. Reno-pericardial orifice. Probe introduced into the left reno-pericardial orifice. g, Non-glandular sac of the left nephridium. h, Glandular sac of the left nephridium. Pore leading from the glandu- lar into the non-glandular sac of the left nephridium. k, Pore leading from the non- glandular sac to the exterior. ac, Anterior. ab, Posterior, cut remnants of the intestine and ventricle. f, It does not contain blood or communicate directly with the blood- I pedal and pleuro-pedal connectives, however, in these cases are only system; this isolation of the pericardium we have noted already in separate in the initial parts of their course, and unite together for the Gastropods and Cephalopods. A good case for the examination of the question as to whether blood enters the pericardium of Lamellibranchs, or escapes from the foot, or by the renal organs when the animal suddenly contracts, is furnished by the Ceratisolen legumen, which has red blood-corpuscles. According to observations made by Penrose on an uninjured Ceratisolen legumen, no red corpuscles are to be seen in the pericardial $ .4 space, although the heart is filled with them, and no such corpuscles are ever discharged by the animal when it is irritated. The pair of renal organs of Anodonta, called in Lamellibranchs the organs of Bojanus, lie below the membranous floor of the pericardium, and open into it by two well-marked apertures (e and f in fig. 18). Each nephridium, after being bent upon itself as shown in fig. 18, C, D, opens to the exterior by a pore placed at the point marked x in fig. i (5) (6). One half of each nephridium is of a dark-green colour and glandular (h in fig. i8). This opens into the reflected portion which overlies it as shown in the diagram fig. i8, D, i; the Cords of three Lamellibranchs. and opens by the pore k to the (From Gegenbaur.) exterior. The renal organs A, Of Teredo. may be more ramified in other B, Of Anodonta. Lamellibranchs than they are C, Of Peden. in Anodonta. In some they a, Cerebral ganglion-pair (=sere- are difficult to discover. That bro-pleuro-visceral). of the common oyster was de- b, Pedal ganglion-pair. scribed by Hoek. Each ne- cates by a narrow canal with the urino-genital groove placed to the front of the great adductor muscle; by a second narrow canal it communicates with the pericardium. From all parts of the pyriform sac narrow stalk-like tubes are given off, ending in abundant widely-spread branching glandular caeca, which form the essential renal secreting apparatus. The genital duct opens by a pore into the urino-genital groove of the oyster (the same arrangement being repeated on each side of the body) close to but distinct from the aperture of the nephridial canal. Hence, except for the formation of a urino-genital groove, the apertures are placed as they are in Anodonta. Previously to Hoek's discovery a brown-coloured investment of the auricles of the heart of the oyster had been supposed to represent the nephridia in a rudimentary state. This investment, which occurs also in many Filibranchia, forms the pericardial glands, comparable to the pericardial accessory glandular growths of Cephalopoda. In Unionidae and several other forms the pericardial glands are extended into diverti- cula of the pericardium which penetrate the mantle and constitute the organ of Heber. The glands secrete hippuric acid which passes from the pericardium into the renal organs. Nervous S :,,stem and Sense-Organs.—I n Anodonta there are three well-developed pairs of nerve ganglia (fig. 19, B, and fig. 1 (6)). An anterior pair, lying one on each side of the mouth (fig. 19, B, a) and connected in of Cyclas. (From sentatives of the cerebral and pleural ganglia Gegenbaur.) of the typical Mollusc, which are not here c, CaQsule. differentiated as they are in Gastropods. A the f e, ciliated cells lining 1p 9,rB, b, andlfig. lo( 6), ax) are theotypical the same. pedal ganglia; they are joined to the cerebro- pleural Otolith. pepleural ganglia by connectives. Posteriorly beneath the posterior adductors, and covered only by a thin layer of elongated epidermal cells, are the visceral ganglia. United with these ganglia on the outer sides are the osphradial ganglia, above which the epithelium is modified to form a pair of sense-organs, corresponding to the osphradia of other Molluscs. In some Lamellibranchs the osphradial ganglia receive nerve-fibres, not from the visceral ganglia, but from the cerebral ganglia along the visceral commissure. Formerly the posterior pair of ganglia were identified as simply the osphradial ganglia, and the anterior pair as the cerebral, pleural and visceral ganglia united into a single pair. But it has since been discovered that in the Protobranchia the cerebral ganglia and the pleural are distinct, each giving origin to its own connective which runs to the pedal ganglion. The cerebro-lower half of their length, or for nearly the whole length. Moreover, in many forms, in which in the adult condition there is only a single pair of anterior ganglia and a single pedal connective, a pleural ganglion distinct from the cerebral has been recognized in the course of development. There is, however, no evidence of the union of a visceral pair with the cerebro-pleural. The sense-organs of Anodonta other than the osphradia consist of a pair of otocysts attached to the pedal ganglia (fig. i (6), ay). The otocysts of Cyclas are peculiarly favourable for study on account of the transparency of the small foot in which they lie, and may be taken as typical of those of Lamellibranchs generally. The structure of one is exhibited in fig. 20. A single otolith is present as in the veliger embryos of Opisthobranchia. In Filibranchia and many Protobranchia the otocyst (or statocyst) contains numerous particles (otoconia). The organs are developed as invaginations of the epidermis of the foot, and in the majority of the Protobranchia the orifice of invagination remains open throughout life; this is also the case in Mytilus including the common mussel. Anodonta has no eyes of any sort, and the tentacles on the mantle edge are limited to its posterior border. This deficiency is very usual in the class; at the same time, many Lamellibranchs have tentacles on the edge of the mantle supplied by a pair of large well-developed nerves, which are given off from the cerebro-pleural ganglion-pair, A ' B a.ad au,y al 'act Balfour.) Both figures represent the glochidium stage. A, When free swimming, shows the two dentigerous valves widely open. B, A later stage, after fixture to the fin of a fish. sh, Shell. ad, Adductor muscle. s, Teeth of the shell. and very frequently some of these tentacles have undergone a special metamorphosis converting them into highly-organized eyes. Such eyes on the mantle-edge are found in Pecten, Spondylus, Lima, Pinna, Pectunculus, Modiola, Cardium, Tellina, Macira, Venus, Solen, Pholas and Galeomma. They are totally distinct from the cephalic eyes of typical Mollusca, and have a different structure and historical development. They have originated not as pits but as tentacles. They agree with the dorsal eyes of Oncidium (Pulmonata) in the curious fact that the optic nerve penetrates the capsule of the eye and passes in front of the retinal body (fig. 21), so that its fibres join the anterior faces of the nerve-end cells as in Vertebrates, instead of their posterior faces as in the cephalic eyes of Mollusca and Arthropoda; moreover, the lens is not a cuticular product but a cellular structure, which, again, is a feature of agreement with the Vertebrate 9 a, Prae-corneal epithelium. f, Retinal nerve. b, Cellular lens. c, Retinal body. d, Tapetum. e, Pigment. g, Complementary nerve. h, Epithelial cells filled with pigment. k, Tentacle. C c, Olfactory (osphradial) ganglion- Phridium in the oyster is a pair. pyriform sac, which communi- by, Byssus. a.ad,Anterior adductor. p.ad,Posterior. adductor. mt, Mantle-flap. bFoot. r, Branchial filaments. au.v, Otocyst. at, Alimentary canal. eye. It must, however, be distinctly borne in mind that there is a fundamental difference between the eye of Vertebrates and of all other groups in the fact that in the Vertebrata the retinal body is itself a part of the central nervous system, and not a separate e (Modified from Horst.) A, Blastulastage(one-cell-layered eaten its way into the in- sac), with commencing in- vaginated endodermal sac, vagination of the wall of the sac at bl, the blastopore. B, Optical section of a somewhat later stage, in which a second invagination has begun—namely, that of the shell-gland sk. bl, Blastopore. en, Invaginatedendoderm(wallof the future arch-enteron). ec, Ectoderm. C, Similar optical section at a little later stage. The invagination connected with the blastopore is now more contracted, d; and cells, me, forming the mcsoblast from which the ceelom and muscular and skeleto-trophic tissues develop, are separated. D, Similar section of a later stage. The blastopore, bl, has closed ; the anus will subsequently perforate the corresponding area. A new aperture, m, the mouth, has N.B.—In this development, as in that of Pisidium (fig. 25), no part of the blastopore persists either as mouth or as anus, but the aperture closes—the pedicle of invagination, or narrow neck of the invaginated arch-enteron, becoming the intestine. The mouth and the anus are formed as independent in-pushings, the mouth with stomodaeum first, and the short anal proctodaeum much later. This interpretation of the appearances is contrary to that of Horst, from whom our drawings of the oyster's development are taken. The account given by the American William K. Brooks differs greatly as to matter of fact from that of Horst, and appears to be erroneous in some respects. modification of the epidermis—myelonic as opposed to epidermic. The structure of the reputed eyes of several of the above-named genera has not been carefully examined. In Pecten and Spendylus, however, they have been fully studied (see fig. 21, and explanation). Rudimentary cephalic eyes occur in the Mytilidae and in Avicula at the base of the first filament of the inner gill, each consisting of a pigmented epithelial fossa containing a cuticular lens. In the Arcidae the pallial eyes are compound or faceted somewhat like those of Arthropods. Generative Organs.—The gonads of Anodonta are placed in distinct male and female individuals. In some Lamellibranchs—for in-stance, the European Oyster and the Pisidium pusillum—the sexes are united in the same individual; but here, as in most hermaphrodite animals, the two sexual elements are not ripe in the same individual at the same moment. It has been conclusively shown that the Ostrea edulis does not fertilize itself. The American Oyster (0. virginiana) and the Portuguese Oyster (O. angulata) have the sexes separate, and fertilization is effected in the open water after the discharge of the ova and the spermatozoa from the females. and males respectively. In the Ostrea edulis fertilization of the eggs is effected at the moment of their escape from the uro-genital groove, or even before, by means of spermatozoa drawn into the sub-pallial chamber by the incurrent ciliary stream, and the embryos pass through the early stages of development whilst en-tangled between the gill-lamellae of the female parent (fig. 23). In Anodonta the eggs pass into the space between the two lamellae of the outer gill-plate, and are there FIG. 24.—Embryo of Pisidfertilized, and advance whilst still in ium pusillum in the diblastula this position to the glochidium phase stage, surface view (after Lan-of development (fig. 22). They may kester). The embryo has be found here in thousands in the increased in size by accumulasummer and autumn months. The tion of liquid between the gonads themselves are extremely outer and the invaginated simple arborescent glands which cells. The blastopore has open to the exterior by two simple ;closed. ducts, one right and one left, continu- ous with the tubular branches of the gonads. In the most primitive Lamellibranchs there is no separate generative aperture but the gonads discharge into the renal cavity, as in Patella among Gastropods. This is the case in the Protobranchia, e.g. Solenomya, in which the gonad opens into the reno-pericardial duct. But the generative products do not pass through the whole length of'the renal tube: there is a direct opening from the pericardial end of the tube to the distal end, and the ova or sperms pass through this. In Area, in Anomiidae and in Pectinidae the gonad opens into the external part of the renal tube. The next stage of modification is seen in Ostraea, Cyclas and some Lucinidae, in which the generative and renal ducts open into a cloacal slit on the surface of the body. In Mytilus the two apertures are on a common papilla, in other cases the two apertures are as in Anodonta. The Anatinacea and Poromya among the Septibranchia are, however, peculiar in having two genital apertures on each side, one male and one female. These forms are hermaphrodite, with an ovary and testis completely separate from each other on each side of the body, each having its own duct and aperture. The development of Anodonta is remarkable for the curious larval form known as glochidium (fig. 22). The glochidium quits the gill-pouch of its parent and swims by alternate opening and shutting of the valves of its shell, as do adult Pecten and Lima, trailing at the same time a long byssus thread. This byssus is not homologous with Is C me bl bl and the cells pushed in with it constitute the stomodaeum. The shell-gland, sk, is flattened out, and a delicate shell, s, appears on its surface. The ciliated velar ring is cut in the section, as shown by the two projecting cilia on the upper part of the figure. The embryo is now a Trochosphere. E, Surface view of an embryo at a period almost identical with that of D. F, Later embryo seen as a trans-m, Mouth. [parent object. ft, Foot. a, Anus. e, Intestine. st, Stomach. tp, Velar area of the prostomium. The extent of the shell and commencing upgrowth of the mantle-skirt is indicated by a line forming a curve from a to F. that of other Lamellibranchs, but originates from a single glandular epithelial cell embedded in the tissues on the dorsal anterior side of the adductor muscle. By this it is brought into contact with the fin of a fish, such as perch, stickleback or others, and effects a hold thereon by means of the toothed edge of its shells. Here it becomes encysted, and is nourished by the exudations of the fish. It remains in this condition for a period of two to six weeks, and during this time the permanent organs are developed from the cells of two symmetrical cavities behind the adductor muscle. The early larva of Anodonta is not unlike the trochosphere of other Lamellibranchs, but the mouth is wanting. The glochidium is formed by the precocious development of the anterior adductor and the retardation of all the other organs except the shell. Other Lamellibranchs exhibit either f a trochosphere larva which becomes a veliger differing only from the Gastropod's and Pteropod's veliger in having bilateral shell-calcifications in-stead of a single central one; or, like Anodonta, they may develop within the gill-plates of the mother, though without presenting such a specialized larva as the glochidium. An example of the former is seen in the development of the European oyster, to the figure of which and its explanation the reader is specially referred (fig. 23). An example of the latter is seen in a common little fresh-water bivalve, the Pisidium pusillum, which has been studied by Lankester. The gastrula is formed in this case by invagination. The embryonic cells continue to divide, and form an oval vesicle containing liquid (fig. 24) ; within this, at one pole, is seen the mass of invaginated cells (fig. 25, hy). These invaginated cells are the archenteron; they proliferate and give off branching cells, which apply themselves (fig. 25, C) to the inner face of the vesicle, thus forming the mesoblast. The outer single layer of cells which constitutes the surface of the vesicle is the ectoderm or epiblast. The little mass of hypoblast or enteric cell-mass now enlarges. but remains connected with the cicatrix of the blastopore or orifice of invagination by a stalk, the rectal peduncle. The enteron itself becomes bilobed and is joined by a new invagination, that of the mouth and stomodaeum. The mesoblast multiplies its cells, which become partly muscular and partly skeleto-trophic. Centro-dorsally now appears the embyronic shell-gland. The pharynx or stomodaeum is still small, the foot not yet prominent. A later stage is seen in fig. 26, where the pharynx is widely open and the foot prominent. No ciliated velum or pre-oral (cephalic) lobe ever develops. The shell-gland disappears, the mantle-skirt is raised as a ridge, the paired shell-valves are secreted, the anus opens by a proctodaeal ingrowth into the rectal peduncle, and the rudiments of the gills (br) and of the renal organs (B) appear (fig. 26, lateral view), and thus the chief organs and general form of the adult are acquired. Later changes consist in the growth of the shell-valves over the whole area of the mantle-flaps, and in the multiplication of the gill-filaments and their consolidation to form gill-plates. It is important to note that the gill-filaments are formed one by one posteriorly. The labial tentacles are formed late. In the allied genus Cyclas, a byssus gland is formed in the foot and subsequently disappears, but no such gland occurs in Pisidium, An extraordinary modification of the veliger occurs in the development of Nucula and Yoldia and probably other members of the same families. After the formation of the gastrula by epibole the larva becomes enclosed by an ectodermic test covering the whole of the original surface of the body, including the shell-gland, and leaving only a small opening at the posterior end in which the stomodaeum and proctodaeum are formed. In Yoldia and Nucula proxima the test consists of five rows of flattened cells, the three median rows bearing circlets of long cilia. At the anterior end of the test is the apical plate from the centre of which projects a long flagellum as in many other Lamellibranch larvae. In Nucula delphinodonta the test is uniformly covered with short cilia, and there is no flagellum. When the larval development is completed the test is cast off, its cells breaking apart and falling to pieces leaving the young animal with a well-developed shell exposed and the internal organs in an advanced state. The test is really a ciliated velum developed in the normal position at the apical pole but reflected backwards in such a way as to cover the original ectoderm except at the posterior end. In Yoldia and Nucula proxima the ova are set free in the water and the test-larvae are free-swimming, but in Nucula delphinodonta the female forms a thin-walled egg-case of mucus attached to 'the posterior end of the shell and in communication with the pallial chamber; in this case the eggs develop and the test-larva is en-closed. A similar modification of the velum occurs in Dentalium and in Myzomenia among the Amphineura.
End of Article: LAMELLIBRANCHIA (Lat. lamella, a small or thin plate, and Gr. /3payxia, gills)
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