FRESH WATER 191 



Freshwater Snails. Creeping along on water plants, on 

 stones, in the mud, or head downwards on the surface there are 

 several different kinds of water-snails, and some of them should 

 be collected for the aquarium. Their movements should be 

 studied and their feeding habits. The spawn of Limncea is often 

 found as a gelatinous mass glued on to the water weeds, and it 

 is interesting to watch the development, and to contrast the 

 state of affairs in this genus with that in Paludina vivipara, 

 which brings forth tiny miniatures of itself. Some are lung- 

 breathers, and these have no lid (operculum) to the shell, namely, 

 Limncea, Planorbis, Physa, andAncylus. Others are gill-breathers, 

 and these have a lid to the shell, namely, Paludina, Bythinia y 

 Valvata, Neritina. As there are not many altogether in Britain 

 this group might be selected for systematic study (there are cheap 

 books like Rimmer's * available). Some forms are easily known by 

 their shells ; thus Planorbis has the shell coiled in a disc, in Physa 

 the transparent shell is coiled to the left, Ancylus has a little 

 simple shell like a nightcap, in Paludina the shell is large, and 

 is usually banded. 



Freshwater Bivalves. Large freshwater mussels species of 

 Anodonta and Unio are often found in ponds, as also in canals 

 and rivers, (i) In the former the shell is thin, and the hinge is 

 toothless ; in the latter the shell is very substantial, and the 

 hinge is toothed. (2) They plough their way in the mud by 

 means of their turgid, ploughshare-shaped muscular " foot/' 

 which corresponds to the flat sole on which snails creep. (3) They 

 feed upon microscopic animals and plants, and the food canal often 

 contains a curious transparent consolidated mass of half-digested 

 food called " the crystalline style." This is easily seen by splitting 

 the foot vertically with a razor after the mussel has been killed and 

 hardened in alcohol. The food is wafted in at the posterior end 

 by the action of millions of microscopic lashes which cover the large 

 plate-like gills, and if a microscope is available a mussel should be 

 sacrificed to show an unforgettable and impressive sight, the ciliary 

 movement. It is simply necessary to cut a little piece off the gill, 

 and put it on a slide, with a drop of water and a cover-glass. 

 (4) There are sometimes pearls fixed to the skin or mantle lining 



1 See (Rimmer) p. 222. 



