VII 



MOLLUSCA 



271 



complex shows interesting differences in position and in the arrangement 

 of its details in the different groups of molluscs. A relatively primitive 

 condition of the pallial complex is seen in the ordinary Siphonopods and, 

 less distinctly, during the young stages of development of ordinary 

 Gasteropods. The deep portion of the mantle-cavity (Fig. 113, A, m.c.) 

 lies on the posterior side of the visceral hump and contains the pallial 

 complex which consists of the anus in the centre (a), the two nephridial 

 openings one on each side of this («), and beyond the nephridial opening 



Fig, 112. 



Nautilus : the left side of the shell has been removed, e, Eye ; h, hood ; 

 p, pupil : s, siphuncle ; s.f, slphonal funnel ; t, tentacle ; v,ft, visceral hump. 



m, mantle-flap 



on each side a gill (ct). The gill, termed in the MoUusca a etenidium, 

 is a feather-shaped organ, with a central axis carrying on each side 

 thin plate - like outgrowths filled with blood which are the actual 

 breathing organs, and are consequently spoken of as the respiratory 

 lamellae. 



In the ordinary adult Gasteropod a remarkable change has come over 

 the pallial complex. In order apparently to escape interference by the 

 foot, which is greatly developed and extends back in such a way that it 

 would obstruct the free inflow and outflow of water, the deep part of 

 the mantle-cavity containing the pallial complex has become shifted 



