POND-SNAILS. 85 
their structure. They inhabit lakes, ponds, and 
ditches of fresh water; are found, but less commonly, 
in rivers, and still more rarely in brackish water. 
They crawl on the mud at the bottom, or on the 
stems and leaves of aquatic plants, always coming 
to the surface, at intervals, to take in a fresh 
supply of air into the lung chamber for respira- 
tion. They may frequently be seen floating, at the 
surface of still water, by the expanded foot, the 
shell being downward and submerged. 
They lay their eggs in round or oval masses ot 
consistent jelly, each mass containing a number of 
egos, varying from three to upwards of a hundred, 
according to the genus. The masses are attached 
to plants or stones beneath the surface, and are 
hatched in about a fortnight after they are de- 
posited. 
The Pond-snails are very numerous, and widely 
distributed, species being found in almost all parts 
of the world. Twenty-four are reckoned by Messrs. 
Forbes and Hanley as British. 
“Jt had been supposed that the shells of flu- 
viatile Mollusca could be distinguished from those 
of the terrestrial kinds, by the edge of the mouth of 
the shell never being furnished with a thickened 
internal rib, and not being in the shghtest degree 
reflexed, and that the animal never closes it with 
an epiphragm; however, further examination has 
shown that when the Pond-snails and the Whirl- 
shells are left nearly dry by the evaporation of the 
water, either by the heat, or by dryness of the 
weather in winter, these animals assume the cha- 
racter of terrestrial Mollusca, thicken and reflect 
their mouth, and form an epiphragm to prevent 
themselves from being destroyed by the drying up 
