THE ANIMALS OF THE FRESHWATER AQUARIUM 103 



again more soberly coloured. Female specimens taken in April, 

 May, or June may generally be relied upon to spawn ; the exact 

 period varies with the altitude, the weather, and the local climate. 



After the Amphibia we come to the fishes, which as animals 

 demanding for the most part well oxygenated water, are never 

 easy to keep in confinement, where the water is always still and 

 tends to stagnate. A few small and hardy forms may, however, 

 be kept with special care. 



Among these we may perhaps begin with the goldfish, a 

 somewhat uninteresting but beautiful form which has been 

 acclimatised to captivity by long generations of breeding. The 

 goldfish is an artificial variety of a species of carp found native 

 in China and Japan, where it has been domesticated for a long 

 period. Accustomed to warmer waters than our climate sup- 

 plies, the goldfish will only breed in warm ponds in hot-houses 

 or similar places. As ordinarily kept in a glass bowl, it is a some- 

 what sluggish and uninteresting creature, which may, however, be 

 usefully employed to demonstrate the points in regard to respira- 

 tion in a fish. The water, as usual in the bony fish, is taken in 

 by the terminal mouth and expelled beneath the gill-cover. As 

 it passes out it washes the gills and purifies the blood contained 

 in them. One would also notice the graceful movements 

 effected by the tail fin, the main organ of locomotion, the paired 

 fins being used in steering movements. All the organs of the 

 body have been, as it were, shifted forwards, and the result is 

 that the tail region is a solid mass of muscle, serving for loco- 

 motion. The contrast between the fins of the fish and the jointed 

 limbs ending in fingers and toes found in the newt or frog, illus- 

 trates one of the distinctions between aquatic and terrestrial 

 animals. As there are no lungs, the nostrils are not used for 

 breathing purposes, and they are mere pits which do not open 

 into the mouth cavity. 



Much more interesting are the sticklebacks, alike from their 

 habit of nest-building and their very remarkable tolerance of 

 both fresh and salt water. The large and interesting fifteen- 

 spined stickleback (Gastrosteus spinachia) is found in the sea 

 or in brackish water (see p. 133), but the three-spined, four-spined, 

 and nine-spined forms all occur in fresh water, though they may 



