138 SEA-SHORE LIFE 
The shell serves not only to protect the intestines, but the en- 
tire head and foot may be withdrawn within it. 
In many sea-snails there is a horny or stony plate called the 
operculum which les on the dorsal side of the posterior end of 
the foot, and when the foot is withdrawn this closes the opening 
of the shell. 
The mantle projects as a curtain beyond the lip of the shell, 
and protects the gills and other organs which le in the space be- 
tween it and the side of the body. Often the mantle curtain is so 
large that it is reflected upward over the outer surface of the shell, 
which it may entirely encase. The secretions of the mantle serve to 
keep the outer surface of the shell smooth and even highly pol- 
ished as in the cowries, but in many cases the shell has degene- 
rated and become permanently covered by the mantle as in slugs. 
In most of the fresh-water snails, and in land snails and slugs, 
the free edges of the mantle have fused with the side of the body, 
leaving only one opening for the admission and expulsion of air on 
the right side of the body. 
In these forms the gill has disappeared, and a sort of lung is 
formed by the ramification of a network of blood vessels over the 
inner surface of the mantle. These snails are obliged to take in a 
fresh supply of air at regular intervals and if one observes a com- 
mon pond snail it will be seen to come to the surface and emit a 
bubble of air from its air-pore and then take in fresh air before 
descending. Indeed, fresh water snails will soon drown if they be 
not allowed to come to the surface to breathe. Curiously enough 
practically none of these lung-breathing snails have an operculum 
when adult. 
The gill of the operculum-bearing snails is feathered and 
close by the side of it one finds a smaller feathered body called the 
osphradium which is possibly an organ for tasting the water that 
is being breathed. In these snails one often finds opposite the gill 
a long tubular fold of the mantle which serves to conduct water 
into the gill chamber. This fold is often protected by a snout-like 
projection of the shell above it. The water is usually discharged 
through another opening which hes farther back. 
The sense organs of the Gasteropoda are poorly developed 
although they are very sensitive to touch. The feelers on the head 
