650 - Heredity and Evolution 



Figure 32-28, all Mollusca, with very rare 

 exceptions, possess one or more shells — which 

 may be chitinous, calcareous, or both. The 

 shell is secreted by a mantle, which, essen- 

 tially, is an outgrowth of the body wall. Also 

 Mollusca have a complete tubular digestive 

 tract and a muscular organ, the foot, which 

 shows great variation in the different classes 

 (Fig. 32-28). In clams and other bivalve mol- 

 lusks, the foot assumes the form of a plough- 

 shaped burrowing organ; among snails and 

 related forms, it is the elongate muscular 

 part that enables the animal to attach itself 



and to crawl along the substratum; whereas 

 in squids and octopuses, the foot is highly 

 modified into a group of powerful tentacles. 

 These tentacles are clad with numerous suc- 

 tion pads, so that the foot may be used for 

 grasping prey, as well as for locomotion 

 (Fig. 32-29). 



The symmetry of these triploblastic, non- 

 segmented animals is essentially bilateral, 

 although some (the snails) display a spiral 

 coiling of the shell and viscera. Also there is 

 a small lined coelom limited mainly (in the 

 adtdt) to the pericardial cavity. 



DIGESTIVE TRACT 



DIGESTIVE TRACT 



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Fig. 32-28. Various mollusks. Note that each displays a foot, shell, mantle, and digestive tube— variously modi- 

 fied in the different kinds. 



