Chapter vii 



THE FRESH-WATER MUSSEL. — CHARACTERS OF THE PHYLUM 

 MOLLUSCA — ENUMERATION OF THE CHIEF PHYLA OF THE 

 ANIMAL KINGDOM. 



In the mussel we meet with an entirely new type of 

 Structure : the animal, like the worm and crayfish, is bi- 

 laterally symmetrical, but there is no trace of metameric 

 segmentation ; the power of locomotion is greatly restricted, 

 and food is obtained passively by ciliary action, as in Infu- 

 soria, not by the active movements of definite seizing organs 

 — tentacles, limbs, or protrusible mouth — as in most of the 

 Metazoa (p. 286). 



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 margaritifera, is found in mountain streams, and other 

 species of the same genus are universally distributed. 



The mussel 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 is seen, the 

 edge of the mantle or pallium. The mantle really consists 

 of separate halves or lobes corresponding with the valves of 



