THE FEESII-WATEK AQUARIUM. 79 



and vice rcrsA. This motion has been supposed by some 

 writers to influence the form of the shell, and most likely it 

 is so. When the young snail is examined under the micro- 

 scope, just before it bursts the egg, we can see through its 

 thin papery shell, and observe the beating of its little 

 heart. The increase in our stock of snails that we may 

 expect, can be imagined, when I state that one of these 

 animals will deposit from two to three of the sacs of eggs 

 a week, thus producing, in the course of six weeks or two 

 months, from 900 to 1,000 young. There are other species 

 of Physa found near New York, one of which, P. pUcata, 

 is obtained from the island on which the city stands. 



The genus Planorbis, or coil-shell, is represented in New 



York State by fourteen species, several of which are com- 



, nion : for instance, P. trivolvis, which I have obtained from 



Oswego Lake. It is said to be abundant in many ponds 



and streams. 



Planorbis bicarinatus (PI. V., Fig. 3) is one of the pret- 

 tiest species of this genus. .It has its three whorls dc- 

 ressed on each side, and is, in appearance, a solid, squared 

 shell. All the Planorbi are fonder of the conferva than of 

 the larger plants, and are, therefore, excellent inhabitants 

 for the fresh-water tank, whilst the Lymnca, most of them, 

 are too dainty to eat up the troublesome minute plants, 

 hut will be sure to make away with your tenderer vege- 

 tables, as Polamogcton. The other common species of the 

 genus Planorbis in New York State are, P. kntus, P. ar- 

 migcrus, which, with P. corpnhntus, are right-handed, 

 whilst the other species P. exae/Uns, P. parvus, arc left- 

 handed shells 



