Chap. 31 



MOLLUSKS -SPECIALISTS IN SECURITY 



645 



Mouth 



Cartilage supporting 

 the radula 



Muscles that 

 rotate radula 



Muscles that retract 

 radula and cartilage 



Fig. 31.12. Upper, holes bored by snails whose rasping tongue (radula) is on 

 the end of a proboscis that is finally pushed into the soft body. They suggest the 

 number of animals consumed by snails and by boring sponges which bore holes by 

 dissolving the shells. Lower, proboscis of marine snail cut lengthwise to show the 

 rasping tongue. {Upper, courtesy, MacGinitie and MacGinitie: Natural History of 

 Marine Animals. New York, McGraw-Hill Book Co., 1949.) 



first as males but later transform into females. This sexual transformation is 

 hereditary, normally occurring in all individuals. 



Class Cephalopoda — Squids and Octopuses 



Characteristics. These are the most highly developed and swiftest of all 

 mollusks (Figs. 31.14, 31.15, 31.16). The head-foot is used equally as a 

 head and a supporting foot. The digestive tract turns back upon itself as it 

 does in snails so that the mouth and anal opening are close together, but there 

 is no such coiling as in snails (Fig. 31.3). In cephalopods, the foot and head 

 with its remarkable eyes are most highly developed and the shell most 



