PULMONARY GASTEKOPODS. 
405 
Fig. 200. Idumea stagimlis 
(Liurnuus). 
^ le snails. This opening dilates and contracts in such a manner as to 
le ceive the air in the cavity, and exclude water when the animal feeds 
1 t , self under the water. The mouth is a trans- 
v erse slit between two rather thin lips, and 
ls armed with small canine teeth. When the 
aill Wal sallies from its shell, it has the appear - 
ailce of a short trumpet. In its interior is a 
r °Undish, thick, and fleshy tubercle, not unlike 
j^e tongue of a paroquet. The true tongue, 
however, which lies at the bottom of the slit, 
18 flat, oval-shaped, and supported by a cartila- 
Ihnous or bony pedicle. 
The Limneans, aided by this very com- 
plicated buccal apparatus, are enabled to feed 
|hemselves with vegetable substances, such as 
he leaves of aquatic plants, which they cut and 
_ r uise with their teeth. They are very active 
111 the season, reproducing towards the end of 
s Pring. At this period little oval or semi-cylindrical masses are fre- 
quently found adhering to floating bodies, glittering and transparent 
as c rystal. These are agglomerations of the eggs of Limneans. When 
'"uter sets in, the Limnea of our climate fall into a state of torpor, 
dll d sink more or less deeply into the mud of the lakes, marshes, rivers, 
° r brooks, which they inhabit. 
These molluscs are of no apparent utility, except to feed fishes and 
dr lUatie birds, which destroy them in great numbers. 
The Planorbis have an organization analo- 
£° Us to the Limneans, of which they are the 
ulif'ul companions in stagnant waters. Their 
. Us (Pig. 201) are thin, light, and disk-like 
111 form, rolled round its plane in such a 
lLUlIl ner as to render all the turns of the spiral 
"sible from above as well as below ; it is con- 
Cave °ir both sides, with an oval, oblong-shaped 
' J Pening, and with an operculum or lid. The 
dIl iraal is conformable to the shell in shape. The visceral mass forms 
‘ j Ver y elongated cone, which unwinds itself absolutely, according to 
e spiral turns of the shell. The foot, or abdominal locomotive mass, 
Fig, 201. Planorbis corncua 
(TJimaius)- 
