UNIVALVE MOLLUSCA. 417 
also collected in gelatinous masses. It passes the winter in a state 
of torpor, buried in the mud of the rivers it inhabits. 
The principal native species is Planorbis corneus (Fig. 188), which 
is common everywhere in Great Britain. 
Another genus of these molluscs, which occupies our fresh-water 
rivers, and often swims with the head down and foot up, is represented 
by the genus Physa. Physa castanea (Fig. 189) has an oval, oblong, or 
nearly globular shell, very thin, smooth, and fragile, opening longi- 
tudinally, narrow above, with the right edge sharp; the last turn of 
the spiral being the largest of all. The animal appears to be inter- 
mediate in form between Planorbis and Limnza; it is oval in form, 
and unrolls itself like the Limnzea, but its tentacles, in place of being 
triangular and thick, like the latter, are elongated and narrow, like 
those of Planorbis. These little inhabitants of the fresh water swim 
with facility, the foot upwards, the shell below, and like Limnzea, they 
feed on vegetables. 
The fourth family, Limacide, containing Testacella and Limax, 
consists of terrestrial pulmonary molluscs, entirely naked, or having 
only a very small shell, The species of the genus Limax vary very 
considerably in appearance, and in consequence of their extreme 
variability we find even individuals of a species differing much. 
When seen creeping along on the surface of the soil, they have nearly 
the form of a very elonglated ellipse, at one extremity of which is the 
head ; the surface of the body in contact with the earth is flat, the 
other convex. Towards the anterior extremity, and upon the middle 
of the back, a portion of the skin projects as if it were detached from 
the body, and is ornamented with transverse stripes of various con- 
volutions. This part is named the cuirass, or buckler, under which 
the animal can hide its head. The mouth is a transverse opening in 
the front of the head; above are two pairs of tentacles, or horns, 
immensely retractile, cylindrical; the lower tentacles are the shorter ; 
the upper terminating in small black points, as in Hedix, which are 
regarded as eyes. 
Upon the right side of the body, and hoilowed in the thickness 
of its edge, which is large and contractile, is the respiratory orifice, 
whose function it is to give access to the atmospheric air, it abuts on 
an internal cavity, also large, which is the pulmonary sac. The outer 
skin, or epidermis, is rayed in brownish furrows, its surface covered 
with a viscous glutinous substance, which permits of the animal 
creeping’ up the smoothest surfaces, locomotion being produced 
by the successive contraction and extension of the muscular fibres 
of the foot. 
BB 
