This progressive fusion of the mantle edges may be taken as indicating definite stages in the development of the Pelecypoda. A perfectly free mantle edge, joined at no point with the edge of the adjacent mantle, occurs in Nucula, Arca, Anomia, and Trigonia (see Fig. [80], A, B). Here there is nothing in the nature of a siphon, either anal or branchial; in other words, no contrivance exists to prevent the spent water which has passed over the branchiae from becoming mixed with the fresh water which is to reach them. When the mantle edges are fused at one point only, this is invariably on the middle part of the posterior side, thus separating off an anal opening which may become prolonged into a tube-like form. At the same time the adjacent underlying portions of the mantle edges draw together, without actually coalescing, to form an opening for the incurrent stream of water, the rudiments of the ‘branchial siphon’ (Fig. [80], C). This is the case with most Mytilidae (see Fig. [75]) with Cardita, Astarte, and Pisidium. In the next stage the branchial opening is separated off by the concrescence of the mantle edges beneath it, and we have the mantle united in two places, thus forming three openings, the ventral of which is the opening for the protrusion of the foot (Fig. [80], D). This is the case in Yoldia, Leda, the majority of the Eulamellibranchiata (e.g. Lucina, Cyrena, Donax, Psammobia, Tellina, Venus, Cardium, Mactra), and all Septibranchiata. In Chama and Tridacna the fused portions of the mantle become more extended, and in Pholas, Xylophaga, Teredo, Pandora, and Lyonsia this concrescence takes place over the greater length of the whole mantle edge, so that the mantle may be regarded as closed, with the exception of the three apertures for the foot and the two siphons (Fig. [80], E).
In certain genera there occurs, besides these three apertures, a fourth, in the line of junction between the pedal and branchial orifices. It appears probable that this fourth orifice (which has been regarded by some as an inlet for water when the siphons are retracted), stands in relation to the byssal apparatus (Fig. [80], F). In Lyonsia, for instance, a thick byssus protrudes through the orifice, which is large and open. In Solen, Lutraria, Glycimeris, Cochlodesma, Thracia, Aspergillum, and a few more genera, which have no byssus, the orifice is very small and narrow. It is possible that in these latter cases, the byssal apparatus having become atrophied, the orifice has been correspondingly reduced in size.[281]
Mantle Reflected over the Shell.—It is sometimes the case that the mantle edges tend to double back over the external surface of the shell, and to enclose it to a greater or less extent. When this process is carried to an extreme, the edges of the reflected mantle unite, and the shell becomes completely internal. We see an incipient stage of this process in Cypraea and Marginella, where the bright polish on the surface of the shell is due to the protection afforded by the lobes of the mantle. A considerable portion of the shell of Scutus is concealed in a similar way, while in Cryptochiton, Lamellaria, and Aplysia the shell is more or less completely enclosed. Among Pulmonata, it is possible that in forms like Vitrina, Parmacella, Limax, and Arion, we have successive stages in a process which starts with a shell completely external, as in Helix, and ends, not merely by enveloping the shell in the mantle, but by effecting its disappearance altogether. In Vitrina and some allied genera we have a type in which the mantle lobes are partly reflected over the shell, which at the same time exhibits rather less of a spiral form than in Helix. In the stage represented by Parmacella, the mantle edges have coalesced over the whole of the shell, except for a small aperture immediately over the spire; the nucleus alone of the shell is spiral, the rest considerably flattened. In Limax the shell has become completely internal, and is simply a flat and very thin plate, the spiral form being entirely lost, and the nucleus represented by a simple thickening at one end of the plate. In Arion, the final stage, we find that the shell, being no longer needed as a protection to the vital organs, has either become resolved into a number of independent granules, or else has entirely disappeared.
Some indications of a similar series of changes occur in the Pelecypoda. The mantle edge of Lepton is prolonged beyond the area of the valves, terminating in some cases in a number of filaments. In Galeomma and Scintilla the valves are partially concealed by the reflected mantle lobes, and in a remarkable form recently discovered by Dall[282] (Chlamydoconcha) the shell is completely imbedded in the mantle, which is perforated at the anterior end by an orifice for the mouth, and at the posterior end by a similar orifice for the anus. In all these cases, except Lepton, it is interesting to notice that the hinge teeth have completely disappeared, the additional closing power gained by the external mantle rendering the work done by a hinge unnecessary. It is quite possible, on the analogy of the Gasteropoda mentioned above, and also, it may be added, of the Cephalopoda and other groups, that we have here indicated the eventual occurrence of a type of Pelecypoda altogether deprived of valves, a greatly thickened mantle performing the part of a shell.[283]
The following works will be found useful for further study of this portion of the subject:—
F. Bernard, Recherches sur les organes palléaux des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7) ix. (1890), pp. 89–404.
G. Cuvier, Le Régne animal (ed. V. Masson); Mollusca, Text and Atlas.
C. Grobben, Beiträge zur Kenntniss des Baues von Cuspidaria (Neaera) cuspidata Olivi, nebst Betrachtungen über das System der Lamellibranchiaten: Arb. Zool. Inst. Wien, x. (1893), pp. 101–146.
E. Ray Lankester, Encyclopaedia Britannica, 9th ed., vol. xvi. (1883), Art. ‘Mollusca.’