THE ANIMALS OF THE FRESHWATER AQUARIUM 107 



To the same family belong the members of the genus Plan- 

 orbis, which have flat, spirally-coiled shells which have been 

 compared in shape to a St. 

 Catherine's wheel. The size 

 varies considerably, the largest 

 forms being about i inch in 

 diameter, while the smallest are 

 minute. Several species live 

 well in captivity, if the aquarium 

 contains a sufficient supply of 

 vegetation for their needs, and 

 they will breed there freely, 

 provided the tank is not 



FIG. 49. Planorbis corneus. 



stocked with carnivorous insects 



or other forms likely to prey upon the young. 



The Gasteropods breathing by gills may be represented in 

 the aquarium by species of Paludina and Bithynia, both hand- 

 some and attractive genera, but both unfortunately only common 

 in southern England ; Paludina, indeed, does not seem to occur 

 in Scotland and Ireland. The species of Bithynia have shells 

 varying from J to J inch in length. The shell is spirally coiled, and 

 conical, and when the animal withdraws into it it closes the 

 aperture behind it by a plate, called the operculum, which just 

 fits the mouth of the shell. The presence of this operculum 

 is a distinction from forms like Limncea and its allies, where no 

 such structure occurs. In B. tentaculatus , the larger and com- 

 moner species, the shell has about six coils ; the animal is to be 

 looked for in slow-moving bodies of water, such as canals and 

 ditches. In captivity it frequently lays eggs on the surface of 

 the glass, and the process should be watched in detail. The 

 eggs are laid in regular bands, the process being very leisurely. 

 They have the usual rounded shape and are like little globes of 

 jelly. 



On the other hand, Paludina retains its eggs within the body 

 till they hatch, and the tiny snails emerge from the body of the 

 parent furnished with a shell resembling that of the adult in 

 spite of its minute size. The adults are very snail-like, the shell 

 being considerably over an inch in length, and nearly an inch 



