4i6 



THE OCEAN WORLD. 



They are of great utility, both to feed fishes and acpatic birds, 

 and also as scavengers of the decaying vegetation of brooks. 



The genus Planorbis has an organisation analogous to Limntea, of 

 which it is the faithful companion in stagnant waters. Their shells (Fig. 

 1 88) are thin, ligjit, 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. i88. — Planorbis corneus 

 (Liniipeus). 



Fig. 187.— Limna;a stagnalis (Linnaeus). Fig. i8g.—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 Limnsea ; 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 



