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APPENDIX B 



reduced or wanting (in the squid, octopus, some snails). Even when the shell is 

 rudimentary or absent, the mantle and its cavity are still present. All Mollusca 

 possess a fleshy organ called the foot, which, in the snail, is usually a flat sole used 

 for creeping over surfaces; in the clam, is generally a wedge-shaped organ used 

 for plowing in mud or sand ; and in the squid, is divided into arms provided with 

 sucking disks and used for seizing prey. 



Fig. B.13. Snails and tooth shells, phylum Mollusca. The tooth shell, Dentalium entale, 

 class Scaphopoda, is in the center above. All the others are snails, class Gastropoda. Upper 

 left, a small whelk, Cyclonassa neritea. Upper right, a conch, Fusus syracusanus. Lower left, 

 a periwinkle, Littorina. Lower right, Helix nemoralis, an air-breathing European tree snail 

 naturalized in the United States. All the others are marine. {Courtesy American Museum of 

 Natural History.) 



Mollusks are among the more abundant of animals. Many snails and slugs 

 crawl about on land, breathing by means of a sort of lung. Fresh-water ponds and 

 streams are the haunts of numerous species of snails, both lung breathers and gill 

 breathers, as well as of the fresh-water mussels and clams. The sea, however, is 

 the home of the greatest variety of mollusks. The principal classes of this phylum 

 are easily distinguished; they are as follows: 



Class 1. The Chitons (Amphineura). These are flattened or wormlike mollusks 

 with obvious bilateral symmetry. The commonest type has the back covered with 



