ia 
et 
hi 
UNIVALVE MOLLUSCA. 415 
the second family of the In-Operculata series. Limnea, Planorbis, 
and Physa, are among the principal genera of this family. 
Species of the genus Limnza live in great numbers in the fresh 
waters of all countries, particularly of temperate climates. They 
cannot remain long under water, being compelled frequently to rise 
to the surface in order to breathe atmospheric air. They are 
even observed, by a mechanism not very well understood, to turn 
upside down, in such a manner as to present themselves feet 
uppermost, and to move slowly along in this position, creeping, as it 
were, through the water. It is difficult to comprehend how the mov- 
able liquid bed upon which the animal operates can offer resistance 
enough to permit of its creeping as if it were ona solid resisting body ; 
but it seems to produce the movement with the assistance of its foot, 
which is broad and thick, and shorter than the shell. 
Limnea is characterised by having a large flat head, from each 
side of which issues a triangular contractile tentacle, carrying at its 
base and on the inner side an extremely small dot, or eye. The 
whole of the somewhat considerable mass of the body is contained in 
a thin diaphanous shell (Fig. 187), the turns in the spiral of which 
are generally elongated, the body whorl being larger than all the 
others, and the aperture is rounded in front. The interior of this is 
occupied by the respiratory cavity, which communicates outwardly by 
an opening analogous to that which exists in the snail. This opening 
dilates and contracts in such a manner as to receive the air into the 
cavity, and exclude water when the animal feeds itself under it. 
The mouth is a transverse slit between two rather thin lips, and is armed 
with small lingual teeth. When the animal sallies from its shell, it 
has the appearance of a short trumpet. The mouth is armed with a 
horny dentated crescent-shaped upper mandible ; the lingual mem- 
brane lies at the bottom of the slit, and is flat, oval shaped, with the 
central teeth inconspicuous, and the lateral ones numerous and 
similar. 
Limnza, aided by its complicated buccal apparatus, is enabled to 
feed on vegetable substances, such as the leaves of aquatic plants, 
which it cuts and bruises with its teeth. They are very active in the 
spring season, towards the end of which they deposit their spawn, 
which at this period is found in the form of little oval or semi-cylindri- 
cal masses adhering to floating bodies, or on stones and aquatic 
plants, glittering and transparent as crystal. When winter sets in, 
the Limnza 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. 
