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THE BOOK OF NATURE STUDY 



breathe by a pulmonary chamber, like a snail or a slug, and 

 must therefore be regarded as terrestrial forms which have gone 

 back to the aquatic life. We have already emphasised the 

 frequency of this condition among freshwater plants and 

 animals, and here, as always, the habit of breathing air, and not 

 air dissolved in water, makes the animals much more tolerant 

 of the special conditions of aquarium life than forms with gills. 



A considerable number of species of Limncea occur in British 

 waters ; the commonest as well as the largest is L. stagnalis, 

 to be found in nearly every stagnant pool or ditch. The shell 

 is thin, spirally coiled, conical in shape, and reaches a length of 

 from i|- to 2 inches. The animal is fond of taking advantage of 

 the surface tension of the water and creeping shell downwards 

 along the surface, with the under part of the muscular foot ex- 

 posed. As it creeps in this way it is easy to see the opening of 

 the pulmonary chamber, which occupies a similar position to 

 that which it does in the snail. The chamber is filled with air 

 at the surface, the animal being as truly an air-breather as is the 

 garden-snail. Eggs are laid freely in captivity. They form 

 little gelatinous masses attached to stones and weed, in the 

 middle of which the little crystal globules which form the eggs 

 proper are clearly seen. 



Related to Limncea, but less common in that they do not 

 extend to the north of these islands, are the species of Anchylus, 



the so-called freshwater limpets. 

 They are not nearly related to 

 the limpets of the sea, and the 

 name is given merely on account 

 of the shape of the shell. The 



47. Bithynia FIG. 48. Freshwater animals are small, and are apt 

 limpet (Anchyius to be overlooked ; the shell 

 rarely much exceeds a \ inch 

 in length. Of the two species, 

 A. lacustriSy the smaller, is found only in still water, while 

 A . fluviatilis, which is slightly larger, occurs in streams. Great 

 care should be taken in detaching the animals from the substance 

 to which they are attached, so as not to damage the delicate 

 shell. 



FIG. 



tentaculata\ notice 

 the operculum clos- 

 ing the shell. 



