BIVALVES. 211 



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 452, of the sea-coast- Take the clam: 

 place it in a large basin of sea-water, and soon it will 

 begin to put out a dark-colored organ as long as the 

 shell, it can stretch it out two or three times the 

 length of the shell. This is supposed by most persons 

 to be the head, but it is not ; the head is within 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 en- 

 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 supply 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 

 currents are caused by a vast number of hair-like 

 fringes which cover the gills within the mollusk, and 

 which are constantly in motion. T^ie position and 

 appearance of the siphonal tubes in fresh-water mus- 

 sels are seen in Figure 437. 



Though mainly small, or of ordinary size, a few bi- 

 valves are very large. In the church of St. Sulpice, 

 in Paris, the valves of a Tridacna weighing five him- 



