THE CLAM AND OTHER BIVALVES 



203 



tentacles occur. Water, bearing food and oxygen, is carried 

 in by the ventral siphon, and undigested substances are 

 carried out by the dorsal siphon. 



The foot of the mussel is large and muscular. It enables 

 the animal to plow its way through mud or even through 

 heavy gravel. The gills and the palps are practically identi- 

 cal in structure in the mussel, the clam, the oyster, and the 

 scallop. With the exceptions just mentioned, the organs have 

 the same general plan of structure in the four animals named, 

 and the description given for 

 the clam applies to the three 

 other forms. There are two ad- 

 ductor muscles in the mussel. 

 The sexes are usually separate. 



Development. All fresh-water 

 mussels, except the small finger- 

 nail shells, carry their young in 

 their gills. When fully formed, 

 the larvae escape from the brood 

 pouch of the female mussel. Fish 

 "nosing" along the river bed 



touch the young mussel, which at that stage is called the 

 glochidium (Fig. 108). The shells of the glochidium clamp 

 together and become attached to the gills or fins of the fish. 

 Some species live only on gills and others on fins. For each 

 species of mussel the larvae will become attached to only 

 certain kinds of fishes. After attachment the tissues of the 

 fish grow around the glochidium, entirely surrounding it. 

 For several weeks the glochidium is transported on the fish. 

 During this time it may be carried into another river, even 

 by way of the sea. When the period of development within 

 the cyst on the fish is completed, the cyst, or covering, 

 breaks away. This liberates the young mussel, which now 

 ceases to be a parasite and begins an independent life. If 



2 l 



Fig. 108. Larva of mussel. 

 (Much enlarged) 



1, shell; 2, adductor muscle; 



3, larval hook ; 4, larval thread. 



(After Balfour) 



