634 



EVOLUTION OF ANIMALS 



Part V 



AMPHINEURA 

 Chiton 



SCAPHOPODA 

 Tooth Shell 



GASTROPODA 

 Snail 



PELECYPODA 

 Clam 



CEPHALOPODA 

 Squid 



Fig. 31.3. Comparison of three important structures in the members of the five 

 classes of mollusks, the shell (heavy line), the digestive tract (solid black) and 

 the foot (stippled). 



Mantle 



Mantle 



isjj;^'" 



Mantle 



+ 



Keyhole limpets 



Chitons, gills withdrawn, 

 gills extended 



^^ 



The eight valves of chiton 



Edge of 

 shell 



Under side on which 

 chiton moves about 



Fig. 31.4. Chiton and keyhole limpets (snails) clamped to a rock lying between 

 the tide lines. Each chiton shows the ancient pattern of overlapping shells that 

 form a flexible roof over its body. When a chiton is turned ventral side up, the 

 foot is exposed, and the borders of the mantle roll up, giving it the common name, 

 sea cradle. 



