CHAPTER VIII 



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. 292). 



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 another kind, 

 Unio pictorum, occurs in many places, and the pearl-mussel, 

 Unto margaritifera, is found in mountain streams, while 

 other species of the same genus are universally distributed. 



The mussel is enclosed in a brown, calcified 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 



