3 66 



MOLLUSC A. 



Gasteropoda it is completely lost. This seems to be in 

 some way associated with the dorsal displacement of the 



Fig. 160. — Nudibranch (Dendronotus arborescens), showing 

 dorsal outgrowths forming adaptive gills. 



viscera in Gasteropods to form the (usually coiled) visceral 

 hump. In Cephalopods there is a somewhat similar dis- 

 placement in a postero-dorsal 

 direction, in Lamellibranchs 

 in a ventral direction, but in 

 neither case is it so marked as 

 in Gasteropods. 



The characters of the inter- 

 nal organs of Mollusca must 

 be gathered from the description 

 of the types, but the nature of 

 the respiratory organs may be 

 briefly noted. Typically, these 

 consist of two feathery gills, 

 sheltered beneath the mantle, 

 and bearing at their bases two 

 Fig. 161.— Ventral surface of osp hradia or smelling patches. 



p„t„iin imlontn. — After Forbes ^,.\. r it . . . , r 



Gills of this typical form occur 



in Cuttles (Nautilus has four), in 



foot, and vascular the simplest Gasteropods (but 



ntle replacing the i /-, i * 



many other Gasteropods have a 

 simple unpaired gill), and in the 

 lowest Lamellibranchs (Solenomya, Nucula, etc.). The respir- 

 atory organs in other Mollusca show much variation when 

 compared with this primitive type. Thus the gills may be 



Paiella vulgata.- 

 and Hanley. 



Note simple eyes at base of tentacles, 

 mouth, median foot, and 

 margin of mant] 

 absent gills. 



