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| Fig. 13.—Transverse Sections of Gill-plates of Anodonta.(After R. H. Peck.) | |
A, Outer gill-plate. B, Inner gill-plate. C, A portion of B more highly magnified. o.l, Outer lamella. i.l, Inner lamella. v, Blood-vessel. | f, Constituent filaments. lac, Lacunar tissue. ch, Chitonous substance of thefilament. chr, Chitonous rod embedded inthe softer substance ch. |
| Fig. 14.—Gill-lamellae of Anodonta. (After R. H. Peck.) |
| Diagram of a block cut from the outer lamella of the outer gill-plate and seen from the interlamellar surface. f, 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 horizontally by the transverse inter-filamentar junctions. |
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| Fig. 15.—Diagram of a view fromthe left side of the animal of Anodontacygnaea, from which the mantle-skirt,the labial tentacles and the gill-filamentshave been entirely removed soas to show the relations of the axisof the gill-plumes or ctenidia g, h.(Original.) | |
a, Centro-dorsal area. b, Anterior adductor muscle. c, Posterior adductor muscle. d, Mouth. e, Anus. f, Foot. g, Free portion of the axis of leftctenidium. h, Axis of right ctenidium. | k, Portion of the axis of the leftctenidium which is fused with thebase of the foot, the two dottedlines indicating the origins of thetwo rows of gill-filaments. m, Line of origin of the anterior labialtentacle. n, Nephridial aperture. o, Genital aperture. r, Line of origin of the posterior labial tentacle. |
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 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 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, the line of concrescence of the inner lamellae of the inner gill-plates is readily seen; it is marked aa in fig. 1 (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 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 by the excurrent or superior siphonal notch of the mantle (fig. 1, d); the lower space communicates by the lower siphonal 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. 1 (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 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 there is least modification by concrescence of the primary filamentous elements of the lamellae.
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| Fig. 16.—Diagrams of Transverse Sections of a Lamellibranch toshow the Adhesion, by Concrescence, of the Gill-Lamellae to theMantle-flaps, to the foot and to one another. (Lankester.) | |
A, Shows two conditions with free gill-axis. B, Condition at foremost region in Anodonta. C, Hind region of foot in Anodonta. D, Region altogether posterior to the foot in Anodonta. a, Visceral mass. b, Foot. c, Mantle flap. d, Axis of gill or ctenidium. e, Adaxial lamella of outer gill-plate. | er, Reflected lamella of outer gill-plate. f, Adaxial lamella of inner gill-plate. fr, Reflected lamella of inner gill-plate. g, Line of concrescence of the reflected lamellae of the two inner gill-plates. h, Rectum. i, Supra-branchial space of the sub-pallial chamber. |
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| Fig. 17.—Vertical Section throughan Anodonta, about the mid-regionof the Foot. | |
m, Mantle-flap. br, Outer, b′r′, inner gill-plate—each composed of two lamellae. f, Foot. | v, Ventricle of the heart. a, Auricle. p, p′, Pericardial cavity. i, Intestine. |
In the 9th edition of this Encyclopaedia Professor (Sir) E. R. Lankester suggested that these differences of gill-structure would furnish characters of classificatory value, and 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 the excurrent 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 (af). 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. 1). 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. 18, A), and giving off an anterior and a posterior artery, and of two auricles which open into the ventricle by orifices protected by valves.
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| Fig. 18.—Diagrams showing the Relations of Pericardium andNephridia in a Lamellibranch such as Anodonta. | |
A, Pericardium opened dorsallyso as to expose the heart andthe floor of the pericardialchamber d. B, Heart removed and floor ofthe pericardium cut away onthe left side so as to open thenon-glandular sac of thenephridium, exposing theglandular sac b, which is alsocut into so as to show theprobe f. C, Ideal pericardium and nephridiumviewed laterally. D, Lateral view showing theactual relation of the glandularand non-glandular sacs ofthe nephridium. The arrowsindicate the course of fluidfrom the pericardium outwards. | a, Ventricle of the heart. b, Auricle. bb, Cut remnant of the auricle. c, Dorsal wall of the pericardiumcut and reflected. e, Reno-pericardial orifice. f, Probe introduced into the leftreno-pericardial orifice. g, Non-glandular sac of the leftnephridium. h, Glandular sac of the leftnephridium. i, Pore leading from the glandularinto the non-glandularsac of the left nephridium. k, Pore leading from the non-glandularsac to the exterior. ac, Anterior. ab, Posterior, cut remnants of theintestine and ventricle. |