VI MANTLE REFLECTED OVER THE SHELL 175 



appearance 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 flat- 

 tened. 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 thicken- 

 ing at one end of the plate. In Ariori, 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 inde- 

 pendent 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 G-aleomma and Scintilla the valves are par- 

 tially concealed by the reflected mantle lobes, and in a remark- 

 able form recently discovered by Dall^ (^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 pos- 

 terior 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 Ce^^halopoda and other groups, that we have here indicated 

 the eventual occurrence of a type of Pelecypoda altogether de- 

 prived of valves, a greatly thickened mantle performing the part 

 of a shell.^ 



The following works will be found useful for further study 

 of this portion of the subject : — 



F. Bernard, Recherches sur les organes j^alleaux des Gasteropodes proso- 

 branches : Ann. Sc. Xat. ZooL (7) ix. (1890), pp. 89-404. 



1 Science, iv. p. 50. 



2 P. Fischer, Journ. cle Conchyh (3) xxvii. p. 20L 



