FIELD BOOK OF PONDS AND STREAMS 



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Fig. 239. — Diagram of one square foot of a pool 

 bottom, showing snails and mussels and associated 

 animals. 



fortably with the simplest arrangements, a glass of water, 

 with a little fresh supply occasionally added, some scrapings 

 of algis, and branches of water plants (Fig. 34). For more 

 permanent living quarters the aquarium should contain four 

 to six quarts of water and have a layer of sand and pebbles 

 on the bottom in which a few branches of Myriopliylliim, 

 Nitella, Elodea, or other water weed have been planted. If 

 some algal scrappings be thrown into the water algag will grow 

 on the glass sides if the snails do not keep it too well cleaned. 

 Snails will live the year round in such an aquarium and will 

 lay their eggs on the glass sides, and the embryo snails can 

 be seen developing within their transparent jelly (Fig. 35). 



Snails. — Gastropoda 



Form and habits of water snails. — Snails are not only numer- 

 ous but conspicuous and familiar. Unlike mussels they travel 

 about in the upper levels of the water where they can be seen; 



304 



