UNIOS AND ANODONS. 103 



the Bylhinia, the mouth of the shell is protected by 

 an operculum, or door. Bythinia is much smaller, and 

 of a more social character, so that we usually find it 

 in great numbers. It also makes a good aquarium 

 object, and lays its eggs in three long rows, usually 

 on stones if there be any, or if not on the stems and 

 leaves of plants. Bivalve shells, such as the little 

 Spharium and Pisidium, may also be safely introduced. 

 Their habits are active, espe- 



Fig. 52. 



cially those of the ubiquitous 

 Sphczrium lacustre, which 

 crawls up and down the tank 

 as if it had only one shell in- 

 stead of two, and occasionally 



Sphcerium corneum. 



indulges itself in a waltz, re- 

 volving at the rate of fifteen or twenty circles a minute ! 

 The tiny pea-shells (Pisidium) are also very abundant, 

 and are extremely useful in an aquarium from their 

 scavenging habit of devouring any dead and decom- 

 posing animals, of which diet they are very fond. If 

 the tank be large enough, and the soil at the bottom 

 sufficiently deep, a specimen of the great swan mussel 

 (Anodonta cygned] might be transferred. Its half- 

 opened shell, showing the tufted syphons, makes it a 

 very pretty and interesting object ; and the student can 

 witness the currents created by the ciliated tubes, by 

 which fresh air and food are taken in, and effete water 

 and matter carried out. A species of Unio might be 

 used instead in smaller tanks, as this is a smaller 



