THE STICKLEBACKS 79 
stones, when the fish, being stunned, fall out.” From this 
not very delicate experiment Aristotle argues that fish can 
hear. 
Twelfth Sub-Order: ACANTHOPTERYGII GASTROSTEIFORMES : STICKLE- 
BACK-LIKE SPINY-FINNED FISHES. 
The fish of this sub-order are distinguished by the dorsal 
fin resolving itself into isolated spines, and by the small mouth 
at the end of a snout generally more or less lengthened. It 
consists of but two families, the sticklebacks and the flute- 
mouths. 
FIRST FAMILY: GASTROSTEIDE: The Sticklebacks 
Body elongated and compressed laterally. Parts of the 
skeleton are produced and expanded to form plates covering 
parts of the skin. There are no scales, and the place of the 
first dorsal fin is occupied by isolated spines. The ventral fins 
are placed on the abdomen, and contain one spine and one 
soft ray. 
The sticklebacks consist of but a single genus, Gastrosteus, 
wherein about ten species have been recognised ; but they are 
such variable creatures that scarcely any two ichthyologists can 
be got to agree as to the number of true species inhabiting 
British fresh waters. Some authorities put it as high as six, 
but Dr. Gunther, who certainly is not unduly scrupulous in 
admitting to the dignity of species certain forms which other 
naturalists refuse to regard as more than varieties, holds that we 
have only three distinct sticklebacks—the three-spined (Gas- 
trosteus aculeatus), the four-spined (G. spinulosus), and the ten- 
spined (G. pungitius). Of these the first-named is by far 
the commonest in this country, and as I am personally un- 
acquainted with the second, I shall do no more than mention 
it; taking occasion to add that it is much to be desired that 
those who have opportunity and inclination should devote close 
