4A, LAND AND FRESH-WATER MOLLUSKS. 
CHAPTER III. 
WATER-BREATHING SNAILS. 
(Gasteropoda.) 
I.—StTRUCTURE AND PHYSIOLOGY OF THE 
GASTEROPODA. 
HE Gasteropods include sea-snails, as the 
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, eachturn 
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 
Fig. 4.—Secti f the shell . 
oN inde, . .. centre, is left, andeaamem 
shells are said to be imperforated ; the axis of the 
shell, around which the whorls are coiled, is 
