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PHYLUM MOLLUSC A. 



Typically the Mollusca are bilaterally symmetrical 

 animals, and this symmetry is marked in the Solenogastres, 

 the Lamellibranchiata, and occurs to a less extent in the 

 Cephalopoda (cf. the unpaired genital organs). In most 

 Gasteropoda it is completely lost. This seems to be in 

 some way associated with the dorsal displacement of the 

 viscera in Gasteropods to form the (usually coiled) visceral 

 hump. In Cephalopods there is a somewhat similar dis- 



FlG. 224. Nudibranch (Dendronotus arborescens], showing 

 dorsal outgrowths forming adaptive gills. 



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 internal 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 Lamellibranchs 

 (Soknomya, Nucula, etc.). The respiratory organs in other 

 Mollusca show much diversity when compared with this 

 primitive type. Thus the gills may be totally suppressed 

 and the mantle may directly take on a respiratory function. 

 This occurs in many marine Gasteropods, for example, in 



