LIMACID^. . 399 



collected in gelatinous masses also. It passes the winter in a state of 

 torpor, buried in the mud of the rivers it inhabits. 



The principal species is Planorbis corneus (Fig. 189) which is 

 common in the rivers of England and France. 



Another group of molluscs, which occupy our fresh rivers, 

 and swim with the shell down and feet up, is represented 

 by Physa castanea (Fig. 190). The genus Phi/so, have 

 an oval, oblong, or nearly globular shell, very thin, smooth, 

 and fragile, opening longitudinally, narrow above, with the F1g- 190> 

 right edge sharp ; the last turn of the spiral being largest oSSe* 



Of all. (LamarckX 



The animal appears to be intermediate in form between Planorbis 

 and Limnsea : 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 fresh water swim with facility, the feet upwards, the 

 shell below, and like Limnsea, they feed on vegetables. 



The fourth family, Limacidse, containing Testacella and Limax, 

 are terrestrial pulmonary molluscs, entirely naked, or having only a 

 very small shell. The Limax varies very considerably in appearance, 

 in consequence of its extreme contractibility. When seen creeping 

 along on the surface of the soil, it has nearly the form of a very 

 elongated 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 convolutions. 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, 

 and terminating in a small button ; the lower tentacles are the shorter ; 

 the upper present at their summit a black point, as in Helix, which 

 have sometimes been mistaken for the eyes. 



Upon the right side of the cuirass, and hollowed in the thickness 

 of its edge, which is large and contractile, whose function it is to give 

 access to atmospheric air, it abuts on an internal cavity, also large, 

 and is intended to promote respiration. The outer skin, or epidermis, is 



