33S MOLLUSCA 



rccallin;,- the sclerotic coat. Two striking peculiarities separate these 

 eyes Irwn those of the vertebrates and show that they have arisen iude- 

 pendentlv aiul have an entirelv dilTerent developmental history, (i) 

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

 anterior cliamber; {2) the layer of rods in the retina abuts against the 

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

 tebrates the reverse is the case. 



Fig. 351. — Sepia offcintilis. the mantle and loft nc]ihriilial sac opened to show the 

 vena cava leading to the liranchial heart. J, anus; />, </, lock of siphon and mantle; 

 g. genital opening; A", head; k. ctenidium; », nephridial sac; 11', nephridial opening; 

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



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

 different Nautilid;B 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 bv the retina, while lens, vitreous hodv, iris, and 

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

 pass through a Xaulilus stage. 



In the truidc anterior and ]iosterior regions are distiitguishable. 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 co^•ered 

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

 periphery of the bodv, often encroaching upoit the back and ;ilways ter- 

 minating ■\\-ith free margins at the head. On. opening the niaittle by a 



