176 ox THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



a deep cleft, and how would the mantle of Firola or Carinaroides 

 answer the definition ? 



If the definition which I have given of the true " mantle " be correct, 

 we must, I think, hesitate for the present in conferring that name upon 

 the dorsal shell-bearing integument of Cliiton. May this not be homo- 

 logous with the thick-edged dorsal surface of a Doris, in which the 

 calcareous particles, instead of being scattered, are united into distinct 

 plates ?i 



With regard to the shells, again, at the risk of being blamed for 

 over-refinement, I would suggest that it is, to say the least, an open 

 question whether the shell of Buccimiin is (as it is commonly supposed 

 to be) homologous with that of Helix ; that of Sepia with that of 

 Nautilus and Annnonites ; that of the embryo Aplysia with that of the 

 adult Aplysia. Grave differences of development occur in some if not 

 in all of these cases. ^ 



From all that has been stated, I think that it is now possible to 

 form a notion of the archet}'pe of the Cephalous Mollusca, and I beg 

 it to be understood that in using this term, I make no reference to any 

 real or imaginary " ideas " upon which animal forms are modelled. 

 All that I mean is the conception of a form embodying the most 

 general propositions that can be affirmed respecting the Cephalous 



^ In D'Orbigny's genus Villiersia, allied to Doris, the calcareous tegumentary particles of 

 the Doridis have united into a flat shell, hidden in the " mantle," which is pierced by the 

 apertures for the tentacles and gills. The disposition of the calcareous particles in the 

 DoridiC is very regular, though it seems too much to assume with Loven that they imitate a 

 subspiral shell (see Loven, Oken's Isis, 1842.) 



^ The memoir by Dr. Gegenbaur, " Beitrage zur Entwickelungsgeschichte der Land Gas- 

 teropoden," which has just appeared in Siebold and Kblliker's "Zeitschrift fiir Wissenschaft- 

 liche Zoologie," furnishes very powerful support to the doubts above suggested, since it 

 demonstrates that the shell of Clausilia, and gives good reason for believing that that of 

 Helix, are developed within the substance of the mantle, following exactly the type of 

 Liniax. 



' ' The land Gasteropoda are distinguished by the peculiar mode of development of their 

 shell, if we may draw conclusions for the whole group from Jfeh'x and Clmisilia. The 

 original deposition of the shell in the interior of the mantle (as in the Loligida among the 

 Cephalopoda) is as yet an isolated fact among the land Gasteropoda, of which we find no 

 indication in other Gasteropods." 



There seems to be a very curious relation between the internal or external nature of a 

 shell, the curvature of its whorls as regards a vertical plane, and the hcemal or dorsal flexure 

 of the intestine. 



Take, first, the case of a true external shell, as that oi Nautilus or Argonatita, or Atlanta. 

 Here the direction in which the shell is wound is the same as that in which the intestine is 

 bent. While the aperture of the shell, therefore, is "hsemad" in Atlanta vi\\\i regard to the 

 axis of columella, it is " neurad" in Nautilus and Argonatita. 



With an internal shell the reverse appears to be the law. Hence the curvature of the 

 shell of Spirula is the opposite of that of the shell of NatUilus, and that of a pulmonated 

 Gasteropod is the same as that of a Pectinibranch. 



