PHYLUM MOLLUSCA 



265 



1. THE PELECYPODA 



A Fresh-water Mussel will serve as a convenient example 

 of the Pelecypoda. 1 Fresh-water mussels are found in rivers 

 and lakes in most parts of the world. Anodonta cygnea, the 

 swan-mussel, is the commonest species in England ; but 

 the pearl-mussel, Unio margaritifer, is found in mountain 

 streams, and other species of the same genus are universally- 

 distributed. Unio complanatus is the common fresh-water 

 pearl-mussel in the Eastern United States, and Anodonta 

 fluviatilis represents the European A. cygnea. 



dp.a 



Fig. 158. — Anodonta cygnea. The entire animal. A, from the left side; B, from 

 the posterior end. d. p. a, dorsal pallial aperture; ex. spk, exhalant siphon; 

 ft, foot; in. sp/t, inhalant siphon; Ig, ligament; m. mantle; urn, umbo. (After 

 Howes.) 



The mussel (Fig. 158) is enclosed in a brown shell formed 

 of two separate halves or valves hinged together along one 

 edge. It lies on the bottom, partly buried in the mud or sand, 

 with the valves slightly gaping, and in the narrow cleft thus 

 formed a delicate, semi-transparent substance (m) is seen, 



1 The earlier and quite as appropriate name of this class is Lamellibran- 

 chiata. — American Editor. 



