100 
NATURAL HISTORY. [NEW BUILDING- 
the water to respire, but they have the power of suspend¬ 
ing their respiration 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 epiphragmci , 
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. 
A. 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cles, can be withdrawn under the skin, which covers them 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. 
1. The family of Arions ( Arionidcz , Case 23) are pecu¬ 
liar for having the end of the tail furnished with a large 
gland, which secretes a quantity of mucus ; this causes the 
end of the body to appear truncated. Their respiratory ca¬ 
vity is on the anterior part of the body, the air hole is in 
the* front of the edge of the mantle, and the orifices of 
reproduction are near this aperture. The shells of the 
animals of this family are in very different degrees of 
developement. Sometimes, as in Avion , when the body 
is simply elongated, it consists of only a few calcareous 
grains; in others, which have the digestive and more 
important organs as it were protruding from the body 
in a small bag-like mantle, this part is covered with 
