PHYLUM MOLLUSCA 



279 



move from place to place, and in the case of some species is 

 permanently fixed to some rock or other solid body by the 

 substance of the larger valve, has no foot. The inhalant 

 and exhalant siphons are sometimes absent, sometimes 

 much longer than in the fresh-water mussel, as in the clam 

 (Mya arenaria). Posterior to the foot there is in 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. 166). In most Pelecypoda the gills (ctenidia, p. 271) 



Fig. 166. — Mytilus edulis, attached by byssus CBy) to a piece of wood. F, foot; 

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



are simpler in character than in the fresh-water mussels. 

 In one group, Protobranchia (JVucu/a, etc.), they take the 

 form of a pair of plume-like organs, and are primitive in 

 shape and structure. 



A remarkably modified member of this class of molluscs 

 is the ship-worm, Teredo, which is very destructive to ships' 

 timbers, piles of jetties, etc. The valves of the shell are 

 extremely small, and the general surface of the mantle 

 secretes a continuous shelly tube lining the burrow in which 

 the elongated worm-like body of the mollusc lies. Highly 



