<Tke FRESHWATER MUSSELS 



L. R. JOHNSTONE 



Unio complanatus 



Drawing h$ Frank L. Tappan 



The Bivalves, to which belong the 

 fresh water mussels, comprise a large di- 

 vision of the Mollusca, the majority of 

 which are marine. The fresh water spe- 

 cies may be found in most rivers, lakes 

 and large ponds, where they plow their 

 way through the mud, feeding on minute 

 unicellular plants and animalcules. Their 

 blood is aerated by means of large gills, 

 through which the water is drawn by 

 dlia or hairs. Sensory papillae are placed 

 around the inhalent aperture — the large 

 opening at the hinder part of the animal. 

 Another small, round opening, the ex- 

 halent aperture, through which the water 

 is returned and refuse expelled, is 

 placed just above the inhalent aperture. 

 Respiration consists in taking the water 

 through the inhaling aperture or syphon 

 and passing it over and between the gills, 

 and thence out of the body through the 

 exhaling opening. These water cur- 

 rents reach the other organs, aerate the 

 blood, convey food to the stomach and 



carry off the waste materials of the sys- 

 tem. 



The reproduction of the freshwater 

 mussels is especially interesting, and illus- 

 trates in a remarkable way the interde- 

 pendence of organisms. The sexes are 

 separate. The eggs are produced in 

 spring and pass into the water-tubes of 

 the gills in such numbers as to distend 

 them like cushions. Here they remain 

 for the first stages of development to take 

 place, which may be completed in two 

 months. The embryos now become qui- 

 escent and pass the winter in the brood 

 pouch. In the following spring they 

 leave the brood pouch and pass out 

 through the upper or exhalant opening 

 into the water. 



The young, which are now known as 

 Glochidia, are small creatures with a bi- 

 valve shell, having a hook on each valve 

 opposite to the hinge, a single muscle to 

 close the valves, and a silk-like thread 

 just behind it. They swim by flapping 

 their valves rapidly, and eventually close 



