124 
NATURAL HISTORY. [jEAST. ZOOL. 
1. 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cles, can be withdrawn under the skin, which covers them 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 bo'dy. 
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 
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, wdiich 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 
a spiral shell, which is generally thin, and sometimes itself 
covered, (when it is polished,) with some reflexed lobes of 
the mantles, as in Helicarion, or nearly naked, with only 
a thick edge to the mantle of the animal, as in Nanina and 
Ste?iopus. 
The family of Snails, Helicidce, have a tapering tail, with¬ 
out any terminal gland. Their pulmonary cavity is generally 
on the front of the back, and the respiratory hole is in 
the hinder part of its margin, while the apertures of the 
reproductive organs are near the base of the right tentacle. 
This family contains many species and genera which are 
difficult to distinguish, from the gradual manner in which 
they pass into each other. A few genera are destitute of 
any shell, or if they have any, it is quite hidden in the 
mantle. Their body is always elongate and attached to the 
foot the whole length. Among these the Limacellus and 
Mekimatium are peculiar for having a large mantle cover¬ 
ing the whole of the back, while the true Slugs, Limax , 
have only a shield-like mantle on the front of the back, 
like the Arions . Most of the genera have a more or less 
exposed shell, which is placed on and protects a thin mem¬ 
branaceous mantle, with a thickened margin, which incloses 
the protruded bag that contains the internal organs. In 
Parmacella , Vitrina , Helicolimax , &c., the margin of the 
mantle is broad, compared to the size of the partly inclosed 
shell, and forms a shield on the front of the body. In 
