397 



LOQHDDJ& 



The Limnseidae, Aquatic Pulmonary Gasteropods, is the second 

 family of the series. They belong to the group that come to the 

 surface of the water to breathe, as do the cetacea and phocas among the 

 Mammifera. The Limndea, Planorbis, and PTiysa are the principal 

 members of this group. 



Limnsea lives in great numbers in the stagnant waters of all 

 countries, particularly of temperate climates. It cannot remain long 

 under water, being compelled frequently to rise to the surface in order 

 to breathe atmospheric air. It is even observed, by a mechanism not 

 very well understood, to turn itself upside down, in such a manner as 

 to present itself feet uppermost, and to move slowly along in this 

 position, creeping, as it were, through the water. It is difficult to 

 comprehend how the movable 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 ; it seems to produce the movement 

 with the assistance of its foot, which is broad and thick, and shorter 

 than the shell. 



Limnsea has 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 most considerable part of the body, 

 comprehending the visceral mass, is spiral, 

 and is contained in a thin diaphanous shell 

 (Fig. 188), the turns in the spiral of which 

 are generally elongated, the last turn being 

 larger than all the others. The interior of this 

 is occupied by the respiratory cavity, which 

 communicates outwardly by an opening 

 analogous to that which exists in the snails. 

 This opening dilates and contracts in such a 

 manner as to receive the air in the cavity, and 

 exclude water when the animal feeds itself 

 under the water. The mouth is a transverse slit between two rather 

 thin lips, and is armed with small canine teeth. When the animal 



Fig. 188. Limnjea stagnalis 

 (Linnseus). 



