416 THE OCEAN WORLD. 
They are of great utility, both to feed fishes and aquatic birds, 
and also as scavengers of the decaying vegetation of brooks. 
The genus Planorbis has an organisation analogous to Limnza, of 
which it is the faithful companion in stagnant waters. Their shells (Fig. 
188) are thin, light, and disc-like in form, rolled round their planes in 
such a manner as to render all the turns of the spiral visible from above 
as well as below; they are concave on both sides, with an oval, oblong- 
shaped opening, which is furnished with a thin peristome. The 
animal is conformable to the shell in shape. The visceral mass forms 
Fig. 188,—Planorbis corneus 
(Linnzeus). 
Fig. 187.—Limnza stagnalis (Linnzus). Fig. 189.—Physa castanea (Lamarck). 
a very elongated cone, which follows the spiral turns of the shell. 
The foot, or abdominal locomotive mass, is short, and very nearly 
round. The head is sufficiently distinct, and furnished with two very 
long filiform, contractile tentacles, having at their base, and on the 
inner side, two small organs, which look like eyes. The mouth is 
armed in the upper part with cross-cutting teeth, and in the lower 
part with a tongue, bristling with a great number of subquadrate and 
bi- or tri-cuspid teeth. 
In habits Planorbis resembles Limnzea ; it creeps like it on the 
surface of solid bodies, and swims in the water with the foot upwards 
and the shell down. It feeds on similar substances, and its eggs are 
