58 COMPARATIVE ANATOMY CHAP. 



or porcelain layer, which, however, has a structure very different from that of the 

 same layer in the Lamellibranchia. This layer is generally, if not always (at least 

 in the young), covered by a periostracum. The inner (nacreous) layer is very often 

 wanting. 



Growth of the Shell. 



In the Arthropoda, the chitinous exoskeleton, which we may 

 compare with the Molluscan shell, develops at the surface of the 

 whole body and its appendages. This skeleton, when once formed 

 and hardened, encases the body on all sides within fixed boundaries, 

 and is incapable of growth. Hence the moults of the Arthropoda, by 

 which alone growth becomes possible. 



The Molluscan shell, on the contrary, is open. In the Gastropoda 

 and Cephalopoda, it assumes the shape of a conical mantle, coiled round 

 a single axis and open at the base of the cone. By continual additions 

 at the edge of its aperture, it grows with the growth of the animal, 

 without materially altering its form. The lines on the surface of the 

 shell of the adult snail register its phases of growth. During growth, 

 the oldest, uppermost coils or whorls of the shell either continue to be 

 filled by the apex of the visceral dome (in many Gastropods^ or are 

 deserted by the animal which, as the shell grows, withdraws farther 

 and farther from its tip. These whorls may remain empty, or may be 

 partially or completely filled with shell substance. In the latter case, 

 they may be successively thrown off. The Nautilus and allied forms, 

 during growth, periodically form transverse septa, so that the forsaken 

 parts of the shell become chambered and filled with gas, 1 the animal 

 occupying the largest and last-formed chamber, which opens externally. 

 In the Lamellibranchia, the growth of the shell keeps pace with the 

 growth of the body in exactly the same manner, the free edge of the 

 shell valve continually receiving additions of shell substance from the 

 edge of the mantle to form the periostracum and the prismatic layers, 

 while the whole external surface of the mantle yields an additional 

 nacreous layer. The consecutive phases of growth are here also 

 registered by the concentric markings on the surface of the shell. 



Special. 



A. Amphineura. (Of. pp. 39-42.) 



B. Gastropoda. 



A few details concerning the shell of the Gastropods must here be added. As a 

 rule, the shell is coiled spirally round an axis. This spiral is, in rare instances, so 

 flattened that the coils come to lie almost' in one plane, giving rise to a nearly 

 symmetrical shell (e.g. Planorbis}. 



There are, however, among the Gastropoda, uncoiled shells which are symmetrical, 

 and these require special attention. The most important are the cup-shaped or more 

 or less bluntly conical shells of the Patellidce and Fissurellidce. Since (1) we derive 



1 Cf. note, p. 37. 



