XuOUHiY FOR MEDICAL STUDENTS CHAP. 



. avity of the shell hut shrinks back from its apical portion,, 



hi tin- Siphonopoda this has become a fundamental 



characteristic of the group. These molluscs have become specialized for 



an a tivr >wimminu existence and the space between the visceral hump 



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



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 tin- body, as in the spongy Cuttle-bone, as it is called, 



ot the ordinary Cuttlefish (Sepia), but the wonderfully archaic Pearly 



Nautilus (\antilns), which though it dates back to extremely ancient 



timrs (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 visa-nil 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 



hell-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 



r 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. in, C) the shell is bivalve. Along the mid- 

 ;! line the substance of the shell undergoes no calcification but con- 

 tinues throughout its thickness to consist of conchiolin which forms an 

 cushion the hinge-ligament connecting together the two halves 

 lives of the shell, lying one to the right and one to the left side of 

 tin 1 : 'ially the two valves are approximately equal, but in 



many IVIrc\p<.da which have adopted a sedentary habit, resting on one 

 'Iocs tin Oyster, the shell becomes lopsided the valve 

 OW being deeper and that which is above flatter. 

 The re-ion of the dorsal surface which forms the mantle extends 

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

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

 -ted a set of important features anus, nephridial 

 and -ilh. toother constituting the pallial complex. The 

 not equally deep all round : it is comparatively shallow 

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



