224 MOLLUSKS: LAMELLIBRANCIIIA. 



These Bivalves have a shell composed of two pieces, 

 or valves, joined together on one side by a hinge, 

 and held tightly together by one or two strong 

 muscles which pass from one valve to the other on the 

 inside. When the animal relaxes these muscles the 

 shell is forced open by an elastic body called a liga- 

 ment, situated at the hinge. Some kinds live in the 

 sea, others in brooks, rivers, ponds, and lakes. Some 

 idea of them all may be gained by studying the Com- 

 mon Mussel, Figure 437, of the brooks, or the Common 

 Clam, Figure 450, of the seacoast. Place the clam in a 

 large basin of sea water, and soon it will begin to put 

 out a dark-colored organ as long as the shell. The 

 Clam can stretch it out two or three times the length 

 of the shell. This is supposed by many persons to 

 be the head, but it is not ; the mouth is ^^'ithin the 

 shell and at the opposite end. At the end of the dark 

 organ are two holes, — one larger than the other, — 

 these being the openings of two tubes which are in- 

 closed in the dark-colored sheath ; and around each 

 opening there is a row of fringes or tentacles. A cur- 

 rent of water is all the time flowing into the larger 

 opening, and another current flowing out of the smaller 

 opening. The first carries in pure water to supph' air 

 to the gills, and minute plants and animals to supply 

 the mouth and stomach with food, and the outgoing 

 current bears away the impure water together with the 

 waste particles which the animal throws off. The cur- 

 rents are caused by a vast number of hairlike fringes 

 which cover the gills within the Mollusk, and which are 

 constantly in motion. The position and appearance 

 of the siphonal tubes in fresh-water Mussels are seen 

 in Figure 435. 



