LAAIELLIBRANCHIA.] MOLLUSCA 691 get the arrangement shown diagrammatically in fig. 135, C, and more correctly in fig. 142. In this region the inner lamellae of the inner gill-plates are no longer affixed to the foot. Passing still further back behind the foot, wo find sfe tf t
- ^f - ~ >JSi - i~: -*, ? . Fio. 138. Gill-lamella: of Anodon (after Peck). A. Fragment of the outer lamella of an inner gill-plate torn from the connected inner lamella, the sub- filamentar tissue also partly cut away round the edges so as to expose the filaments, their transverse junctions tr, and the "windows" left in the lattice work ; sfe, internal surface of the lamella ; v, vessel. B. Diagram of a block cut from the outer lamella of the outer gill-plate and seen from the inter- lamellar surface (after Peck). /, constituent filaments ; trf, fibrous tissue of the transverse inter-filamentar junctions ; v, blood-vessel; ilj, inter-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 horizon tally by the transverse inter-filamentar junctions. in Anodon the condition shown in the section D, fig. 135. 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 A & fuse by concres cence at g. In the lateral view of the animal with reflected mantle - skirt and gill - plates, the line of concrescence of the , n ,. Fio. 130. Transverse sections of A, a Lamellibranch, inner lamellae OI and S, an Isopleurous Gastropod (Chiton), to show the inner rill- ^ ie regions of j>, the foot; br, the branchiae; and & in, the mantle. (From Gegenbaur.) plates is readily seen; it is marked aa in fig. 124, (5). In the same figure the free part of the inner lamella of the inner gill-plate resting on the foot is marked z, whilst the attached part the most anterior has been snipped with scissors so as to show the genital and nephridial apertures x and y. The concrescence, then, of the free edge of the reflected lamellae of the gill-plates of Anodon is very extensive. It is important, because such a concrescence is by no means universal, and does not occur, for example, in Mytilus or in Area ; further, because when its occurrence is once appreciated, the reduction of the gill-plates of Anodon to the plume-type of the simplest ctenidium presents no difficulty; and, lastly, it has import ance in reference to its physio logical significance. The me chanical result of the concres cence of the outer lamelke to the mantle-flap, and of the inner lamelke to one another as shown in section D, fig. 135, is that the sub-pallial space is divided into two /</ ] / P spaces by a horizontal Sep- Fio. HO. Lateral view of a Mactra, turn. The upper space (i) communicates with the outer world by the excurrent or su perior siphonal notch of the mantle (fig. 124, d); the lower space communicates by the lower siphonal notch (e in fig. 124). the right valve of the shell and right mantle -flap removed, and the si phons retracted, br, br , outer and inner gill-plates ; t, labial tentacle ; to, tr, upper and lower siphons ; ms, siphonal muscle of the mantle-flap ; ma, anterior adductor muscle ; m;>, posterior adductor muscle ; p, foot ; c, umbo. (From Gegeiibaur.) The only communica tion between the two spaces, excepting through the trellis- work of the gill-plates, is by the slit (z in fig. 124, (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 indirectly through the pores of the gill-plates, that the water introduced into the lower sub-pallial space must pass on its way to the excurrent siphonal notch. Such a subdivision of the pallial chamber, and direction of the FIG. 141. The same animal as fig. 140, with its foot and siphons expanded. Letters as in fig. 140. (From Gegenbaur.) currents set up within it do not exist in a number of Lamellibranchs which have the gill-lamellae comparatively free (Mytilus, Area, Trigonia, tc.), and it is in these forms that there is least modification by concrescence of the pri mary filamentous elements of the lamellae. Probably the gill -structure of Lamellibranchs will ultimately furnish some classificatory characters of value when they have been thoroughly investigated throughout the class. The alimentary canal of Anodon is shown in fig. 124, (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 the excurrent stream passes. The coil of the intestine in Anodon is similar to that of other Lamellibranchs, but the crystalline style and its diverticulum are not present here. 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 (of). The food of the Anodon, as of other Lamellibranchs, consists of microscopic animal and vegetable organisms, which are brought to the mouth by the stream which sets into the sub-pallial chamber at the lower siphonal notch
(e in fig. 124). Probably a straining of water from solid
