338 



MOLLUSCA 



recalling the sclerotic coat. Two striking peculiarities separate these 

 eyes from those of the vertebrates and show that they have arisen inde- 

 pendently and have an entirely different developmental history, (i) 

 The cornea in most Decapoda has an opening by which water enters the 

 anterior chamber; (2) the layer of rods in the retina abuts against the 

 vitreous body and the ganglionic layer lies behind, while in the ver- 

 tebrates the reverse is the case. 



FIG. 351. Sepia officinal is, the mantle and left nephridial sac opened to show the 

 vena cava leading to the branchial heart, ti, anus; b, d, lock of siphon and mantle; 

 g, genital opening; A", head; k, ctenidium; n, nephridial sac; ', nephridial opening; 

 sp, nephrostome; /, ink sac; Tr, siphon. 



The foregoing description applies to but part of the Cephalopoda. The very 

 different Nautilidas have, instead of tentacles with suckers, numerous shorter 

 tentacles on lobular appendages, which are developed differently in the two 

 sexes. The eyes are deep pits, opening to the exterior by a small aperture, the 

 base of the pit being occupied by the retina, while lens, vitreous body, iris, and 

 cornea are lacking (fig. 350). It is to be noticed that the other cephalopod eyes 

 pass through a Nautilus stage. 



In the trunk anterior and posterior regions are distinguishable, the 

 two passing into each other on the sides. The anterior (which corre- 

 sponds only in part to the ventral side of other molluscs) is wholly covered 

 by the mantle, a strong muscular fold, which takes its origin from the 

 periphery of the body, often encroaching upon the back and always ter- 

 minating with free margins at the head. On opening the mantle by a 



