270 ZOOLOGY FOR MEDICAL STUDENTS chap. 



the complete cavity of the shell but shrinks back from its apical portion, 

 leaving a space. In the Siphonopoda this has become a fundamental 

 characteristic of the group. These molluscs have become specialized for 

 an active swimming existence and the space between the visceral hump 

 and the apex of the shell becomes filled with gas secreted into it which 

 serves to reduce the specific gravity of the creature to approximately 

 that of the water in which it lives, so that it does not have to expend 

 muscular energy in constant exertion to keep itself from sinking. In 

 most of the existing Siphonopods the shell has become greatly modified 

 and enclosed within the body, as in the spongy Cuttle-bone, as it is called, 

 of the ordinary Cuttlefish {Sepia), but the wonderfully archaic Pearly 

 Nautilus {Nautilus), which though it dates back to extremely ancient 

 times (Silurian period) still lingers on to the present day in the Indian 

 and Pacific Oceans, shows us what the Siphonopod shell was primitively 

 like. It is a long cone coiled into a flat spiral. During its formation 

 the apex of the visceral hump periodically shrinks back from the shell 

 and then, having come to rest for a time, proceeds to cover itself with 

 a layer of shell which forms as it were a floor to the deserted portion of 

 the shell-cavity. The result is that in the fully developed Nautilus the 

 cavity of the shell consists of a series of chambers separated from one 

 another by the successive floors or partitions (Fig. 112). It is only the 

 last or outermost of these chambers that is filled by the visceral hump. 

 The others are occupied merely by secreted gas, except that a thin 

 prolongation of the visceral hump, called the siphuncle (Fig. 112, s), is 

 continued through them and the intervening partitions. 



In the Pelecypoda (Fig. iii, C) the shell is bivalve. Along the mid- 

 dorsal line the substance of the shell undergoes no calcification but con- 

 tinues throughout its thickness to consist of conchiolin which forms an 

 elastic cushion — the hinge-ligament — connecting together the two halves 

 or valves of the shell, lying one to the right and one to the left side of 

 the body. Usually the two valves are approximately equal, but in 

 many Pelecypoda which have adopted a sedentary habit, resting on one 

 side of the body, as does the Oyster, the shell becomes lopsided — the valve 

 that is below being deeper and that which is above flatter. 



The region of the dorsal surface which forms the mantle extends 

 normally on to a kind of flap or skirt — the mantle-flap — hanging down 

 all round and covering in a recess (the mantle-eavity or pallial cavity) 

 in which are situated a set of important features — anus, nephridial 

 openings and gills, together constituting the pallial complex. The 

 mantle-cavity is not equally deep all round : it is comparatively shallow 

 except in the neighbourhood of the pallial complex, and this pallial 



