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 

 bilaterally symmetrical, but there, is no trace of meta- 

 meric segmentation ; the power of locomotion is greatly 

 restricted, and food is obtained passively by ciliary 

 action, as in Infusoria, 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, Unio 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-trans- 

 parent substance is seen, the edge of the mantle or 

 pallium. The mantle really consists of separate halves 

 or lobes corresponding with the valves of the shell 



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