194 METHODS IN TEACHING 



active water bugs, add much to the interest in the aquarium 



by their rapid movements and by their 

 Water Boatmen .. ,, , . , f . 



_, , . peculiar method of carrying: a bubble of air 



Backswimmers r 



with them for respiration. Under water, 

 this has much the color and brightness of a drop of mer- 

 cury. 



Pond snails are easily obtained. The eggs, deposited on 

 the glass or on stems of plants, are valuable for showing 



active cell division. Mount some in a lit- 

 Pond Snails 



tie water in a watch crystal, and examine 



with a microscope of low power. If the eggs are taken 

 when quite fresh, an hour or two after they are deposited, 

 the beginning of segmentation can be followed for the first 

 ten hours, or until the cells become too small to be seen 

 with the low power. Later, the developing eye and heart 

 can be made out, and still later, the motion of the young 

 snail itself in the egg. These snails are found in permanent 

 ponds, watering troughs, and streams. Daphne, cy clops, 

 and other " water fleas " are very small crustaceans, num- 

 bers of which are to be found in temporary rain ponds in 

 spring. They add interest and furnish food for other 

 forms of life, looking out for themselves and requiring no 

 attention on the part of those who care for the aquarium. 



If muck from the bottom of a permanent pond or of a 

 sluggish stream is used on the floor of the aquarium instead 



of the layer of coarse sand or gravel, or if 



Freshwater , . . . , ., , . . 



Mussel muck is mixed with this regular cover- 



ing, a small specimen of the fresh water 



mussel can be kept. 

 With the first heavy rains of winter and spring the adult 



salamanders appear in considerable numbers in the ponds. 



