UNIVALVE MOLLUSC A. 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 Limnaea ; it is oval in form, 

 and unrolls itself like the Limnaea, 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 Limnaea, they 

 feed on vegetables. 



The fourth family, Limacidae, 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 Helix, which are 

 regarded as eyes. 



Upon the right side of the body, and hollowed 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. 



o 



