THE BOOK OF NATURE STUDY 



also extend into tidal pools. The fifteen-spined form is very 

 different from its allies in appearance , but any stickleback may 

 be recognised by the presence of a variable number of spines in 

 front of the dorsal fin, by the more or less elongate and compressed 

 body, and by the oblique mouth cleft. They are hardy forms, and 

 can be kept in an aquarium with the usual precautions. The 

 three freshwater forms (named G. aculeatus, G. spinulosus, and 

 G. pungitus respectively) may be recognised by the number of 

 the spines in front of the dorsal fin. The three-spined form is 

 by far the commonest, and shows well the large plates at the sides 

 of the body which are one of the characteristics of the genus. 

 In the breeding season the male takes on bright colours, and 

 constructs a nest in which the female lays the eggs. The nest 

 is made of weeds and twigs, woven together by slimy threads. 

 It is shaped like a tiny barrel, and has openings at each end to 

 permit the passage of currents of water over the eggs. More 

 than one female often lays her eggs in the same nest, and these 

 are watched with the utmost zeal by the male, who ventilates 

 them by fluttering his pectoral fins so as to cause a current of 

 water. The females, on the other hand, appear perfectly in- 

 different to the fate of the eggs, and will even devour them. 

 As the males are exceedingly pugnacious, only one should be 

 kept in the tank at a time. The common stickleback varies in 

 length from about two to four inches, and is thus about the same 

 size as the other very common small freshwater fish, the minnow 

 (Leuciscus phoxinus). The last-named is of the same family 

 as the goldfish. It may be distinguished from the sticklebacks 

 by its imbricating scales and the absence of spines. The lateral 

 line, or row of sense organs at the side of the body which is char- 

 acteristic of fish, is here incomplete. Eels are also hardy in 

 captivity, and small specimens form admirable inhabitants of the 

 aquarium, where their graceful movements are well worth 

 watching. The little bull-head or miller's thumb (Coitus gobio), 

 with its huge, unwieldy head, its gaping mouth, and its large 

 eyes, may also be added to the aquarium, but its voracious 

 appetite makes it a not very desirable acquisition. In general 

 also it must be admitted that fish are not easy to keep alive for 

 long in the aquarium as it is available in the schoolroom, and the 



