124 INTRODUCTION TO ZOOLOGY CHAP. 



must be shaded with a curtain. The floor of the tank should be 

 covered with a layer, one or two inches deep, of small stones, which 

 have been well washed by stirring them in running water, until 

 the water that passes off is quite clear and bright. In this gravel, 

 some healthy plants must be established before any animals are 

 put into the tank. When snails are to be kept, it is well to put 

 in plenty of some very common weed, such as the Canadian water- 

 weed, as the snails eat a great deal, and would soon destroy rarer 

 and more delicate plants. This weed will grow floating freely in 

 the water, but it is best to tie it down with cotton to a stone, in 

 bunches of seven or eight pieces, and then bury the stone in the 

 gravel at the bottom of the tank, so that the branches of weed 

 stand up in the water. When the weeds have been planted and 

 the tank filled with water, the snails may be introduced. 1 



The snails may be obtained from any pond in which 

 tae&oafls ^ iere 1S pl en ty of vegetation, by drawing a fishing- 

 net through the weeds. They may be carried home 

 in a tin ; if plenty of water- weed is put in to keep them damp, no 

 further water is necessary for a few hours. 



Suggestions ^e snai l s should be watched, sketches made of 

 for practical them in several different views, and their habits noted. 

 Work. After they have been in the tank a little while, their 

 egg-masses may be found. The day should be noted on which 

 one such mass is deposited, and a record kept of the history 

 of the development of the eggs. If there are fish in the tank, 

 the weeds to which the egg-masses are attached should be removed, 

 and kept in a shallow dish of water apart, for fish will feed greedily 

 on the young snails as they hatch. Different batches of eggs 

 should be kept under different conditions as to warmth and the 

 volume of water in which they are reared, and the effect of the 

 variations on their rate of development noted. 2 The young snails 

 should be given plenty of free-floating filamentous Algae for 

 food. 



Periwinkles, Cowries, Limpets, Dog Whelks, and Dog Periwinkles 

 may be kept in the sea-water tank described at the end of Chapter 

 III., but only one or two at a time, and care must be taken that 

 the water is kept well aerated. Periwinkles are very apt to 

 escape, so the tank must be kept covered ; their predilection for 

 leaving the water should, however, be considered, and the rock- 

 work in the tank built up above the water surface. Dog whelks 



1 Pond water or clean rain water is the best to use, but ordinary tap water 

 usually answers quite well, unless exceptionally hard. 



2 Compare Semper's results given in Animal Life, Int. Sci. Series, vol. 

 xxxi. pp. 161-64. 



