XI 



PHYLUM MOLLUSC A 



277 



tunity of becoming attached to the gills or skin of a fish. 

 Fixed firmly by means of the hooked valves the larva 

 remains as an external parasite for about ten weeks, 

 becoming encysted by an over-growth of the skin or mucous 

 membrane of its host. In the meantime a metamorphosis 

 is taking place, and when the young Mussel becomes free it 

 has begun to assume the form and structure of the adult. 



The majority of the members of the class Pelecypoda 

 resemble the Fresh-water Mussel in the main features above 



B. 



_.-sh 



sm. 



FIG. 153. A, advanced embryo of Anodonta. B, free glochidium. f. provisional 

 byssus ; j. shell ; sh. hooks ; snt. adductor muscle ; so. sense organs ; w. cilia. 

 (From Korschelt and Heider's Embryology.} 



described. They are bilaterally symmetrical, laterally com- 

 pressed, and possess a mantle, consisting of paired right and 

 left lobes, which secretes a bivalved calcareous shell. A 

 distinct head is never present. On the ventral surface is 

 a muscular foot; there are two adductor muscles, and 

 two pairs of gills. But, on looking over a collection of 

 shells of various bivalves, it will be found that certain cf 

 them differ from that of the Fresh-water Mussel in not 

 having the two valves of the shell alike. This inequality 

 between the two valves of the shell is strongly marked in 



