THE BOOK OF NATURE STUDY 



To the observation of the last-named (Gastrosteus aculeatus\ 

 which occurs in different varieties in different waters, many 

 profitable hours may be devoted. Let us utilise the lore of 

 the schoolboy who knows about the voracity of both sexes and 

 the pugnacity of one, the male's change of colour from silvery 

 white to brilliant reds and other colours at the breeding season, 

 his building of the rough nest, and his enticing of the female to 

 enter it, and his zealous parental care for the eggs and the young. 

 One should look for the thin sticky threads (from the kidneys) 

 with which the male birds bind together the materials of the 



nest ; one should 

 watch the develop- 

 ment of the young 

 fry ; one should test 

 the fact that the 

 fishes can endure a 

 transition to sea 

 water. Sticklebacks 

 are dangerous addi- 

 tions to a cherished 

 aquarium, unless one 

 wishes to study the 

 survival of the fittest, 

 the relatively fittest, 

 but they can be kept 

 by themselves and 



FlG. 115. Six stages in the development of the trout. 2, 

 shows the process of hatching ; 3 and 4, show the large 

 yolk-sac. 



fed and watched. We should look for the bony shields on the 

 sides of the body, the three depressible dorsal spines, the stout 

 spine on the pelvic fins, the single spine in front of the unpaired 

 ventral fin, and so on. After one has made sure of the stickle- 

 back it will be time to tackle the minnow. 



Another very profitable thing to do is to lift carefully out of 

 the water the long leaves of flags and reeds and the like, placing 

 them for examination in a basin of water. In this way we often 

 get water-snails (e.g. species of Lymnceus) ; small leeches (Nephelis 

 octoculata), sometimes a dozen together on one leaf ; still simpler 

 " worms " like Planaria ; the elegant cases of some of the small 

 caddis-flies ; the freshwater Hydra ; and bell-animalcules like 



