105 
the last, for the valves being nearly hid in the mantle ROOM XIII. 
of the animals. n at . hist. 
Cases 53 — 60 contain the fifth class of Gasteropodous 
Mollusca: they respire free air, which is received into 
a cavity between the mantle and the back, lined in¬ 
ternally with numerous reticulated vessels. They are 
mostly terrestrial, and when aquatic, they come to the 
surface of the water to respire; but they have the 
power of suspending their respiration, for a consider¬ 
able time during the cold of winter, and in the dry sea¬ 
son, in warm climates. 
The tentacula of the kinds which always live on land, 
are retractile into themselves, like the finger of a glove, 
as in the family of the slugs (. himacida, ’), case 55 ; which 
have either only a few calcareous grains in the mantle, 
as in the genus Arion ; or a small shell imbedded in this 
part, as in the Limax; or a small ear-shaped shell placed 
over the mantle at the end of the body, as in Testacella. 
The family of snails ( Helicidce ), have a large shell, into 
which the animal can withdraw itself, as in the genera 
Helix , Bulimus, Partula , Pupa , Vertigo , and Clausilia. 
These have the edge of the mouth thickened, and 
often toothed ; others, as Succinia , Acliatina , Zonites, 
Namnia, and Vitrina, have the edge of the mouth thin: 
the two last have the front of the mantle produced be¬ 
yond the mouth of the shell, like a shield, and a lobe at 
its side, which partly covers the shell, and gives it the 
polished coat for which they are peculiar. 
Those which live in water, have subulate, con¬ 
tractile tentacula, with the eyes on their inner base, as 
in the family of Auriculidce, containing the genera 
Auricula and Melampus . and others, have their eyes 
near their outer base, as in the family of Pond Snails , 
( Lymnceadce.) Some of these, as the Lymncea , have the 
inner lip of the shell plaited; while the others, as the 
Physce and Planorbes , have the shell constantly re¬ 
versed, or with the whorls turning from the left to the 
right, contrary to their direction in most other shells;—- 
and the Ancyli , which have simple conical shells, hav- 
ing 
