FRESH-WATER MUSSEL 



373 



this similarity in larval forms as an example of adaptive parallelism 

 in a similar type of environment. Certainly, morphological evidence 

 shows a close relationship. 



Classification of this phylum is based on the nature of the foot, 

 and respiratory organs; shape and structure of the shell; arrange- 

 ment and structure of the nervous and reproductive systems. 



Class I. Amphineura 



Includes the Chitons, vvliich are found abundantly on rocks between 

 tide marks along the Atlantic and Pacific Coasts. Bilaterally 

 symmetrical body; tentaculess head, eyes absent; shell, if present, 

 consists of eight overlapping plates. Most species have a flattened 

 foot but other species are slender and wormlike. (Ischnochiton 

 conspicuus.) 



Class II. Pelecypoda 



Includes the bivalve mollusks, such as the oysters, clams, scallops, 

 and cockles. More than ten thousand species have been described, 

 of which approximately four-fifths live in the ocean. Division of 

 the class into orders is based on gill characters. (Clams.) 



Class III. Gastropoda 



Includes the snails and slugs. Approximately fifty-five thousand 

 species have been discovered and described. Shell, if present, 

 univalve. (Snails.) 



Class IV. Scaphopoda 



Marine. Mantle edges grown together along ventral side forming 

 tube, with a shell of same shape and open at both ends. Commonly 

 known as tooth shells. Approximately 300 known living species. 

 (Dentalium.) 



Class V. Cephalopoda 



Marine. The most highly organized of the mollusks. A definitely 

 formed head is present which bears a pair of eyes that superficially 

 resemble the eyes of vertebrates. The foot is modified into arms 

 or tentacles. They are carnivorous animals and many of them are 

 used as food by man. (Nautilus, LoUgo, Polypus.) 



Habitat and Behavior of the Clam 



Mussels or clams are usually found partly buried in the mud, 

 sand or gravel of ponds, lakes, or streams. By means of the mus- 

 cular foot which is protruded from between the two valves at the 

 anterior end of the shell they plow their way slowly through the 

 -stream or pond bed, feeding on the microscopic organisms in the 



