UNIVALVE MOLLUSCA. 4I 5 



the second family of the In-Operculata series. Limncea, Planorbis, 

 and Physa, are among the principal genera of this family. 



Species of the genus Limnsea 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 doAvn, 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 on a 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. 



Limnaea 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. 



Limnaea, 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 cr}'stal. When winter sets in, 

 the Limnaea 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. 



