154 



MOLLUSCA 



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, we find 



FIG. 138. Gill-lamellse 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 

 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 _ 



, ,. - Fro. ISO. Transverse sections of A, a Lamellibranch, 



inner lamellae Ot and B, an Isopleurous Gastropod (Chiton), to show 

 fh inner trill t' ie relations of j>, the foot; !-, the branchiae ; and 

 ' m, 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 lamellae to 

 the mantle-flap, and of the 

 inner lamellae to one another 

 as shown in section D, fig. 

 135, is that the sub-pallial 

 space is divided into two 



spaces by a horizontal sep- FIG. HO. Lateral view of a Mactra, 

 tum. The Upper Space (i) 'e nght^valve of the shell and right 



communicates with the outer 

 world by the excurrent or su- 

 perior siphonal notch of the 

 mantle (fig. 1 24, d) ; the lower 

 space communicates by the 

 lower siphonal notch (e in fig. 124). 



man tie -flap removed, and the si- 

 phons retracted. &r, &r', outer and 

 inner gill-plates ; t, labial tentacle ; 

 ta, tr t upper and lower siphons ; ms, 

 siphonal muscle of the mantle-flap ; 

 ma, anterior adductor muscle ; mp, 

 posterior adductor muscle ; p, foot ; 

 c, umbo. (From Gegenbaur.) 



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 (<?) 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, ifec.), 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 

 (a/). 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 



