254 



THE RISE OF ANIMAL LIFE 



chiton 



snail 



clam 



squid 



Fig. 12-3. Modifications in the body plan of various kinds of mollusks. 



for a retreat in case of danger. Although a 

 clam is usually safe in its tightly closed 

 shell, it is nevertheless preyed upon by the 

 starfish, which has the ability to open the 

 shell and devour the soft body parts (Fig. 

 12-24). Some members of the phylum have 

 no shells, such as the slug and octopus. 

 They are protected only by their coloration, 

 their habits, or the ability of some to dis- 

 charge a cloud of inky material into the 

 water which totally obscures them. 



The digestive tract is tubular, much the 

 same as in the annelids, although it is coiled 

 in various ways in the different groups of 

 the mollusks (Fig. 12-3). Many of these 

 animals are provided with a peculiar rasp- 

 ing tongue, the radula, which is found no- 

 where else in the animal kingdom. It is used 

 in loosening algae from surfaces, and in 

 tearing bits of plants loose as the animal 

 feeds. The radula is a long ribbon of tough 

 tissue, to which many sharp teeth are 

 attached. Muscles are arranged so as to 



pull the radula back and forth over a pro- 

 jection which is thrust out through the 

 mouth while feeding. It is an interesting 

 and clever device to facilitate feeding. 



The clam 



The fresh-water clam, although differ- 

 ing in some respects from other molluscan 

 forms, is a familiar representative of the 

 entire phylum. It is a bilaterally symmetri- 

 cal, "headless" animal, enclosed in a double 

 shell, usually found partly buried in the 

 sand of lakes or streams. By means of its 

 hatchet-shaped, muscular foot, which pro- 

 trudes from the shell, it is able to plow 

 slowly along, feeding on microscopic forms 

 of Hfe. 



When the clam "walks" the foot is thrust 

 forward between the two valves of the 

 shell. This permits blood to flow into the 

 many sinuses of the foot, causing it to swell 

 and thus form an anchor. As the retractor 

 muscles contract, the clam is drawn for- 



Fig. 12-4. Locomotion of a clam. 



