182 LAND AND FRESH-WATER MOLLUSKS. 
mollusks. Their shells are extremely variable, 
spiral and turreted, dextral or sinistral, discoidal 
and limpet-like ; delicate, fragile, and of a uni- 
form colour. They have no operculum, but 
secrete a thin epiphragm when the pools, in 
which they live, dry up. 
They inhabit fresh-water, at a small depth, 
as they are compelled to rise frequently to the 
surface to breathe; many of them can lower 
themselves in the water by a glutinous thread. 
They lay their eges in a’ transparent jelly mass 
on the leaves and stems of water plants, or on 
stones. The early development of a Pulmo- 
niferous snail can be most conveniently observed 
in a [imneea. In an aquarium, the eggs are 
often deposited on the sides of the glass, and in 
that case a microscope can be brought to bear 
on the object, without at all interfermg with the 
course of nature. The egg sacs are dull at the 
moment of laying, but after remaining in the 
water a few hours they become transparent. 
The form of the albuminous mass varies some- 
what with the species ; it is usually an elongated 
oval. The eggs are hatched in about 30 days. 
The snail is usually infested with a species of hair 
worm (Gordius inquilinus), numbers of which 
attach themselves around the neck and beneath — 
the tentacles, and are ever vibrating. They are 
falsely-parasitic, for they appear to derive no 
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