gallery.] natural HISTORY. 127 
ihochetes is peculiar for having a bundle of bristles placed 
on each side of the valves ; and Chitonellus and Amicula 
only differ in having the valves nearly hidden in the man¬ 
tle of the animals. 
The remaining Gasteropodous Mollusca have been called 
Pneumobranchiata, from their respiring free air, which 
is received into a cavity between the mantle and the back, 
lined internally with numerous reticulated vessels, like 
the lungs of beasts and birds. 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 re¬ 
spiration for a considerable time during the cold weather 
of temperate and the dry season of warm climates. They 
are all oviparous, but in a few the eggs are hatched before 
they are deposited. 
I. The greater part of the kinds of this order have the 
respiratory cavity closed by the edge of the mantle being 
attached to the back of the neck, leaving only a small 
hole, covered with a fleshy valve, for the entrance and 
escape of air. These are all hermaphrodite, and they are 
always destitute of any true operculum, but the mouths 
of their shells are closed, during the period of their tor¬ 
pidity, with a temporary lid, called the epiphragma , 
which is formed by the hardened juices of the body 
moulded on the surface of the contracted animal; this lid, 
which is easily removed by the pressure of the foot, when 
its edge has been softened by the mucous secretions of the 
animal within it, is renewed as often as the animals require 
it. 
1. The more terrestrial kinds have the eyes seated at 
the top of the long cylindrical tentacles, which are placed 
above the true ones. The latter are sometimes rudiment¬ 
ary, or even wanting. 
In some of these the head, eye-pedicels, and tenta¬ 
cles, can be withdrawn under the skin, which covers them 
like a sheath, into the cavity of the body. They have a 
single crescent-shaped horny upper jaw, which is often 
toothed on the edge. The stomach is single and mem¬ 
branaceous. 
The family of Arions ( Arionidce ) are peculiar for 
having the end of the tail furnished with a large gland, 
which secretes a quantity of mucus; this causes the end 
