MOLLUSCS 109 
V.—MOLLUSCS. 
INTROPUCTION.—Molluses are soft-bodied animals, many of 
which secrete a hard protective covering calleda.shell. A feware 
air-breathers like tlie Garden Snail; but most of them obtain a 
supply of oxygen by means of gills. 
The molluses may be divided into two main sections distin- 
guished by the presence or absence of a distinctly marked head. 
The forms having a head correspond to those whose shells consist 
of a single valve, whilst the headless forms are distinguished 
by the possession of a bi-valve shell. To the former belong the 
Gastropods (Snail, etc.), and to the latter the Lamellebranchs, 
(Oysters, Mussels, ete). 
LAND MOLLUSCS. 
The Snail. The Garden Snail is a pulmonary or air- 
breathing Gastropod. The shell is in one piece, spirally 
B 
Fig. 70.—A, Shell of Garden Snail. 
a, spire; b, body whorl. 
B, Garden Snail withdrawn from Shell and crawling along. 
coiled, and consists of a number of whorls. The portion of 
the shell in which the animal lives, is known as the body- 
whorl, whilst the rest of the shell is called the spire. As 
the animal grows larger, the body whorl increases in size 
by additions made to its aperture. 
The snail has a well marked head, which bears in front 
two pairs of retractile tentacles or horns. The tips of 
the upper pair, which are longer and thicker than the lower 
pair, are furnished with eyes, 7.¢,, there is an eye at the 
