FRESH WATER 



193 



which is most wanted in school, partly because some of the facts 

 can be readily verified, and partly because it may serve to show 

 that a pond will supply pro- 

 blems enough and wonders 

 enough to keep " Nature 

 students " at work and in ad- 

 miration for all time coming. 

 It is not necessary to go to the 

 tropics or the deep sea. 



The small freshwater bi- 

 valves (Cyclas or Sphcerium 

 and Pisidium) which are very 



- ' n . _, / FIG. 109. Dytiscus beetle, male and female. 



common and differ obviously 



from the mussels in being very small and in having siphons, keep 

 their young within the mother until they are fully formed. 



The larva of 

 the widespread 

 Dreissen si a 

 differs from that 

 of all other fresh- 

 water bivalves 

 in being free- 

 swimming. 

 Water-Beetles. 



FlG. 1 10. Larva of Dytiscus marginalis much enlarged. There is for 



tunately available in Miall's Aquatic 

 Insects l a very convenient aid and 

 an inspiring stimulus to a study 

 which is rich in rewards to those 

 who will pursue it patiently. We 

 cannot do more here than refer to a 

 few of the commonest water insects. 

 " One of the most abundant and 

 common beetles throughout the 

 kingdom " is the whirligig or steel- 

 coat (Gyrinus natator) , a lively little 

 insect about a quarter of an inch long, 



1 See (Miall) p. 222. 

 VOL. ii. 13 



FIG. i IT. Larva of Dytiscus 

 marginalis seizing a tadpole. 



