FRESH-WATER MUSSELS. 31 



ing this border, it will soon be found that the border forms 

 two openings which lead into the shell. 



Great care must be taken not to jar the dish, or the table 

 upon which it rests. The slightest jar will cause the shells to 

 instantly close. If some indigo, or small particles of dirt, be 

 dropped near these openings, currents of water will be re- 

 vealed ; one current pouring out of the opening nearest the 

 back, and another current as steadily pouring in at the other 

 opening. The opening into which the current of water is 

 passing is called the incurrent orifice, while the orifice from 

 which a current of water is passing is called the excurrent 

 orifice. The incurrent orifice is sometimes called the respira- 

 tory orifice, because the water is taken in to supply the gills 

 which are the breathing or respiratory organs of the mussel, 

 and this orifice corresponds to the siphon in the sea snails 

 already studied. This current of water, besides bathing the 

 gills, also carries in minute particles which are floating in 

 the water, and these particles are conducted to the mouth of 

 the creature, and swallowed as food. At the opposite end of 

 the shell from these openings, or the forward end, a whitish, 

 fleshy mass will be seen protruding. This is called the foot, 

 and corresponds to the foot or creeping disk in the snails. 

 By means of this foot the mussel crawls through the sand. 



The mouth is above the foot, and always concealed within 

 the shell. In Fig. 35 the foot is shown, and also the excur- 

 rent and incurrent orifices, with arrows drawn to indicate 

 the direction of the currents of water. 



In some small species of fresh-water bivalves, the excur- 



