FRESH -WATER MOLLUSK^. 569 



Now hold a perfect mussel-shell in your hand (that is, a mussel in 

 which both valves are together, and united across the hack), with the 

 ligament uppermost, and the umbone away from the person, or beyond 

 the ligament, and the valve on the left hand is the one which covers 

 the left side of the animal, while 'the valve on the right hand covers 

 the right side of the animal. The forward end will be the end away 

 from the person, and the hinder end will, of course, be the end toward 

 the person. {See Fig. 1.) 



Let us now endeavor to collect some fresh-water mussels alive. 

 These may be found partly buried in the sand or mud of rivers and 

 lakes. As they crawl along partly buried in this way, they plough up 

 the sand, leaving a well-marked furrow or groove behind them. Every 

 boy that goes in bathing is familiar with the peculiar furrow left by 

 the fresh-water mussel. By following such a furrow, the mussel that 

 made it will soon be found. 



Fig. 8 represents the appearance of a common fresh-water mussel 

 in the act of crawling. 



Fig. 8. Suowing Position of Mussel vmim cra-^tling. 

 /, Foot buried below the Surface of the Sand s. Above the line s is supposed to be water, the line 



representing the bottom of a lake or river. 



Having collected a few in this way, they may be placed in a large, 

 shallow pan of water, and allowed to remain quiet for a while. Grad- 

 ually the shells will open a little, and from the hinder end a curious 

 fringed border appears ; on examining this border, it will soon be 

 found that it 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 revealed ; one current pour- 

 ing out of the opening nearest the back, and another current as stead- 

 ily pouring in at the other opening. The opening into which the cur- 

 rent of water is passing is called the incurrent orifice, while the orifice 

 from wliich a current of water is passing is called the e?i'current orifice. 

 The incurrent orifice is sometimes called the respiratory orifice, because 

 the water is taken in to suj^ply the gills which are the breathing or 



