44 
LAND AND FRESH-WATER MOLLUSKS. 
CHAPTER III. 
WATER-BREATHING SNAILS. 
( Gasteropoda .) 
I.—Structure and Physiology oe the 
Gasteropoda. 
T HE Gasteropods include sea-snails, as tlie 
whelk, limpet, &c., and a few fresh-water 
snails, as the common marsh snail (Paludina 
vivipara), which may be studied as a type of the 
latter. 
Shell . — All the fresh-water snails live in a 
single or univalve shell, having the form of a more 
or less conical spiral; it may be regarded as a tube 
wound upon itself, each turn 
of which is called a whorl 
or volution ( a ), fig. 4; the 
lines of junction of the 
whorls are called sutures 
(b ); by the close coiling of 
the whorls, a pillar of shell, 
or columella (c), in the 
centre, is left, and such 
shells are said to be imperforated ; the axis of the 
shell, around which the whorls are coiled, is 
Fig. 4.—Section of the shell 
of Paludina. 
