70 
NATURAL HISTORY. 
[EAST. ZOOL. 
ing to the pulmonary cavity, which is placed on the hinder part of the 
right side, is at the hinder extremity of the body, between the edge of 
the mantle and the foot. They live in forests under leaves. 
The family of Onchidiam: are very like the former, but the 
back is warty, and they have only two contractile rather club-shaped 
tentacles, and a broad lunate head; the respiratory organs are posterior, 
just under the edges of the mantle. The Onchidia live on aquatic 
plants in ditches, and Onchis and Peronia under stones on the sea shore. 
B. The aquatic kinds, on the contrary, have the head and tentacles 
of the same structure and surface as the rest of the body, and like it, are 
only contractile; the eyes are always placed on the side of the base of 
the tentacles. They are divided into two families: the Auriculidce and 
Pond Snails, or Limnceadce. 
The family of Aurtculidhd (Case 26) are peculiar for having 
cylindrical tentacles, like the land slugs, and their eyes are placed on 
the inner side of the base of these tentacles. They have a ringed 
conical muzzle. The mantle is thin, with a thickened edge ; they are 
always provided with an external spiral shell, which has a plaited pillar 
in all ages; and the animal has the peculiarity of absorbing the septa 
which separate the cavities of the whorls from one another, even in 
Scarabus, which has these parts only incompletely developed. The 
true Auriculce have a thickened edge to the mouth of the shell, which 
is covered with a brown periostraca. The Scarabus , like JRanella , 
forms half a whorl between each period of rest, the thickened and re¬ 
flexed part of the lips forming an edge to each side of the shell. The 
Sidulce have a sharp internal ridge to the outer lip. Carichium is one 
of the smallest British shell-bearing Mollusca; it has a sinuated mouth 
and a reflexed lip, like a Bulimus. The Conovuli, which are found 
under stones on the sea-shore, and in the mud of salt marshes, have an 
obconic shell with a narrow linear mouth; and the Chilince , which live 
in clear running streams, in South America, have much the habit of the 
Pond Snails, from the shells of which they are chiefly to be distin¬ 
guished by the sharpness of the plaits on the pillar, and by the shell be¬ 
ing spotted. 
The family of Pond Snails (Limn^ad^e, Case 26) differ from the 
former in having compressed tentacles, with the eyes at the outer side of 
their base ; their muzzle is short and dilated at the end; their shell is uni¬ 
formly horn-coloured, with a more or less oblique plait on the pillar; 
they live on vegetables, having a muscular stomach. Like many marine 
Gasteropodes, they have the power of floating on the surface of the 
water, with the back dowmwards, the concave surface of the foot forming 
a kind of boat; their eggs are transparent, and deposited in oval masses 
on water plants. The outer lip of the shell is thin, but when the ponds 
are dried up, the animal strengthens it by an internal rib, and forms a 
membranaceous epiphragma over the mouth, like the land-snails. The 
shells are generally external, as the Limncece and Aplexi; the former 
having an oval shell and triangular tentacles, and the latter filiform ten¬ 
tacles and a reversed shell, like the Planorbes , which differ in having a 
diseoidal depressed shell, with the whorls revolving one over the other 
on their own axis. The Physce bear the same relation to the Aplexi 
as the Amphipeplece to the Limncece , but they each have the edge of 
the mantle produced and reflexed over their thin polished shell w r hen 
