398 THE OCEAN WORLD. 



sallies from its shell, it has the appearance of a short trumpet. 

 In its interior is a roundish, thick, and fleshy tubercle, not un- 

 like the tongue of a paroquet. The true tongue, however, which lies 

 at the bottom of the slit, is flat, oval-shaped, and supported by a 

 cartilaginous or bony pedicle. 



Limnsea, aided by this very complicated buccal apparatus, is enabled 

 to feed itself with vegetable substances, such as the leaves of aquatic 

 plants, which it cuts and bruises with its teeth. They are very active 

 in the season, reproducing towards the end of spring. At this period 

 little oval or semi-cylindrical masses are frequently found adhering to 

 floating bodies, glittering and transparent as crystal. These are 

 agglomerations of the eggs of Limneea. When winter sets in, the 

 Limnsea of our climate fall into a state of torpor, and sink, more or 

 less deeply, into the mud of the lakes, marshes, rivers, or brooks, which 

 they inhabit. 



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

 and also as scavengers of the decaying vegetation of brooks. 



Planorbis has an organization analogous to Limnsea, of which it is 

 the faithful companion in stagnant waters. Their shells (Fig. 189) 

 are thin, light, and disk-like in form, rolled round its plane in such a 

 manner as to render all the turns of the spiral visible from above as 

 well as below ; it is concave on both sides, with an oval, oblong-shaped 

 opening, and with an operculum or lid. The animal is conformable 

 to the shell in shape. The visceral mass forms a very elongated cone, 

 which unwinds itself absolutely, according to 

 the spiral turns of the shell. The foot, or 

 abdominal locomotive mass, is short, and very 

 nearly round. The head is sufficiently dis- 

 tinct, and furnished with two very long 

 filiform, contractile tentacles, having at their 

 base, and on the inner side, a small organ, 

 Fig. ie9. pianorbis corneus which looks like an egg. 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 hooked excrescences. 



In habits Planorbis resembles Limnaea : 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 



