272 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



forwards round the right side of the body towards the head (Fig. 113, B). 



The pallial complex is now anterior to the visceral hump and it has 



become reversed in position^ its original 

 right side being now on the left. The 

 pallial complex has also lost its original 

 symmetry, the original left ctenidium 

 having disappeared while the original left 

 nephridium has lost its original function 

 and now serves as the genital duct. The 

 persisting ctenidium (Fig. 113, B, ct) also 

 commonly loses its original symmetry, 

 the respiratory lamellae disappearing from 

 its left side so that it takes the shape of 

 a comb rather than that of a feather. In 

 some of the more familiar Gasteropods — 

 v^he snails and slugs — which have taken to 

 breathing air, the deep part of the mantle- 

 cavity functions as a lung. Its original 

 opening to the exterior has been narrowed 

 down to a small round hole on the right 

 side (Fig. iii, A, p.a); the ctenidium has 

 disappeared and its respiratory function 

 has been taken over by a rich network of 

 blood-vessels developed over the inner 

 surface of the mantle. In some of our 

 marine Gasteropods living between tide- 

 marks such as the common little Peri- 

 jvinkles or " Wilks " {Litiorina) the first 

 steps in a similar evolution may be seen, 

 for although the ctenidium is still present 

 and functional a rich network of blood- 

 vessels for aerial respiration has developed 

 in its neighbourhood. 



In the Pelecypoda, while the pallial 

 complex retains its primitive symmetry, 

 it has become much changed in other 

 respects through the great modification 

 of the ctenidium. The Pelecypoda are 

 molluscs which have given up active 



browsing or predatory habits and have taken to subsisting on minute 



particles of food material floating in the water such as bacteria 



a 



Fig. 113. 

 Plan of pallial complex as seen from 

 above. A, primitive arrangement ; 

 B, Gasteropod ; C, Peleoypod. The 

 inner boundary of the mantle-cavity is 

 indicated by a dotted line, a, Anus ; 

 ct, ctenidium ; g, genital opening ; 

 m,c, deep part of mantle - cavity ; 

 «, nephridium. 



