CHAPTER TV 



MOLLUSCA 



Phylum TROCHOZOA 



Sub-Phylum MOLLUSCA 

 Class AMPHINEURA 

 GASTEROPODA 

 SCAPHOPODA 



PELECYPODA. . . Anodonta 

 CEPHALOPODA 



THE Mollusca include the eight-shelled chitons and their 

 allies (Amphineura) ; the univalves, or snails, slugs, buckies, 

 etc. (Gasteropoda) ; the tooth shells (Scaphopoda) ; the 

 bivalves, as oysters, clams, mussels (Pelecypoda) ; and 

 octopuses, squids, and cuttle-fish (Cephalopoda). 



Although bivalve Mollusca are common in the sea, there 

 are many species which are found only in fresh water. These 

 have been grouped under several allied families, of which the 

 Unionidae is important from the economic value of species 

 of Unio. This family is world-wide in distribution, but pre- 

 dominates in the south. 



Anodonta. The fresh-water mussel, Anodonta cygnea, is 

 common in rivers and in ponds of the northern hemisphere, 

 and is spread over Europe, Asia, and North America. The 

 genus is found also in South America. It is so closely similar 

 in structure to other members of the Unionidae that the 

 following description will' enable the dissection of any species 

 to be readily followed. There is little essential difference 

 between it and the large siphonate species so common on mud 

 flats and in shallow bays in the sea, as the clam, Mya arenaria, 

 and the gaper, Lutraria elliptica. 



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