THE SLUG 215 



ditches and muddy or stagnant ponds. Its shell runs up 

 into a sharp spire, and is right-handed, i.e. holding the 

 shell so that its aperture is next the observer and below, the 

 aperture is at the right (Fig. 202). Limneea crawls over the 

 bottom, up the stalks of aquatic plants, and on the surface 

 film of water. It comes to the surface, indeed, with great reg- 

 ularity, thrusts its lung-opening out into the air and breathes, 

 and then it rapidly retreats into the water. It is a remark- 

 able fact that an air-breathing organism, whose ancestors were 

 aquatic and gill-breathing, should, after having once become 

 emancipated from the water and having accjuired lungs, have 

 returned again to the water. Indeed, some Limngeas have 

 become so adjusted to an aquatic life that they can live at 

 the bottom of deep lakes and never come to the surface to 

 breathe. In the case of such snails the lung is permanently 

 filled with water ; the oxygen dissolved in the water being 

 used in respiration. The Limnseas that live in shallow 

 ponds are deprived of water in times of drought. They 

 then burrow deep in the mud and close the aperture of their 

 shell. By this means the loss of the water of the body is pre- 

 vented and the existence of the animal is prolonged until the 

 return of the rainy season. 



Physa ^ has a smaller, relatively stouter shell than Limnisa, 

 and one whose coil is left-handed (Fig. 202). It lives in even 

 the smaller ponds and brooks, and may be easily reared in 

 aquaria. It feeds freely upon any kind of vegetable matter. 

 Physa heterostropha is the common species of the United 

 States (Fig. 202, left). 



Planorbis ^ is coiled in one plane like a watchspring.^ It 



' physa, bellows. - planus, flat ; orbis, circle. 



3 Fig. 203. 



