GENERAL NOTES ON MOLLUSCS. 



395 



Gasteropoda it is completely lost. This seems to be in 

 some way associated with the dorsal displacement of the 





^N*7-*I- 



"ip 



" X > *- ~~> "v ,5.-- "^ * ; 41 



-^~-. "- - a t. 



->. ^^^^ ~~-^ ,, i 



FIG. 196. Xudibranch (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 inferred from the description 

 of the types, but the nature of 

 the respiratory organs may be 

 briefly noted. Typically, these 

 consist of two feathery gills, or 

 ctenidia, with an axis attached to 

 the body and bearing a double 

 row of lamellae. These are 

 sheltered beneath the mantle, 

 and bear at their bases two 

 osphradia or smelling patches. 

 Gills of this typical form occur 

 in Cuttles (Nautilus has four), 

 in the simplest Gasteropods (but many other Gasteropods 

 have a simple unpaired gill), and in the lowest Lamelli- 

 branchs (Sole/wmya, Nuciila^ etc.). The respiratory organs 



FIG. 197. Ventral surface of 

 Patella vulgata. After Forbes 

 and Hanley. 



Note simple eyes at base of tentacles, 

 mouth, median foot, and vascular 

 margin of mantle replacing the 



absent gills. 



