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MANUAL OF ZOOLOGY 



SECT. 



the Scallops or Clams, and even more so in the Oysters. 

 The Clams and Oysters are also examples of Pelecypods 

 which have only one adductor muscle instead of two, and the 

 Oyster, moreover, has no foot : it is unable to move from 

 place to place, and in the case of some species is perma- 

 nently fixed to some rock or other solid body by the sub- 

 stance of the larger valve. The inhalant and exhalant 

 siphons are sometimes absent, sometimes much longer than 

 in the Fresh-water Mussel. Posterior to the foot there is in 



FIG. 154. MytilUS edlllis, attached by byssus (By.) to a piece of wood. F, foot ; 

 S, exhalant siphon. (From the Cambridge Natural History.) 



many Pelecypods a gland termed the byssus gland, secreting 

 silky threads which serve to attach the animal temporarily or 

 permanently as, for example, in the Sea Mussel (Mytilus) 

 (Fig. 154). In most Pelecypoda the gills, or ctenidia, as 

 they are usually termed, are simpler in character than in the 

 Fresh-water Mussels. In one group, the Protobranchia, they 

 take the form of a pair of plume-like organs. 



A remarkably modified member of this class of Molluscs 

 is the Ship-worm, Teredo (Fig. 155), which is very destructive 

 to ships' timbers, piles of jetties, etc. The valves of the 



