2 7 8 



MANUAL OF ZOOLOGY 



cysted by an overgrowth of the skin or mucous membrane 

 of their 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 

 described. They are bilaterally symmetrical, laterally com- 

 pressed, with a mantle consisting of paired right and left 

 lobes, secreting a bivalved calcareous shell. A distinct head 

 is never present. There is on the ventral surface a muscu- 



p.-- 



FlG. 165. — A, advanced embryo of Anodonta ; B, free glochidium. /.provisional 

 byssus ; s, shell ; sh, hooks ; s??i , adductor muscle ; so, sense organs ; iu, cilia. 

 (From Korschelt and Heider's Embryology.) 



lar foot ; there are two abductor muscles, and there are two 

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

 various bivalves, it will be found that certain of 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 the scallops, 

 and even more so in the oysters. The oysters are also 

 examples of Pelecypods, which have only one adductor 

 muscle instead of two ; and the oyster, which is unable to 



