414 PHYLUM MOLLUSCA. 
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- 
Fic. 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 (4Vawéi/us has four), in the 
simplest Gasteropods (but many other Gasteropods have a 
simple unpaired gill), and in the lowest Lamellibranchs 
(Solenomya, 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 
