364 FISHES CHAP. 



valence of some form of vocal organ. According to Sorensen, the 

 first mentioned of the three families includes no less than 68 

 species, which utilise the air-bladder alone as a sound-producing 

 organ. Nevertheless, there still remain many Teleostean families, 

 rich in genera and species, and with an almost world-wide 

 geographical distribution, in which such organs have not yet 

 been found. 



The advantages which Fishes derive from the possession of 

 sound-producing organs are sufficiently obvious. 



A characteristic feature in the reproduction of most Fishes is 

 the general absence of any process of conjugation between the sexes, 

 the eggs being fertilised in the water after their extrusion from 

 the body of the female, and, consequently, any device which will 

 facilitate the formation of shoals during the breeding season must 

 be of great advantage to the species by largely increasing the 

 chances that the ova will be fertilised, and thus secure the more 

 successful propagation of the race. Hence it may be concluded 

 that the vocal organs of Fishes are a means to this end, and that 

 the sounds they produce are in fact recognition -sounds which 

 enable Fishes of the same species to congregate together at 

 periods when reproductive activity is greatest. This view is 

 in harmony with much that is known of the habits of these 

 Fishes, especially with the fact that particular sounds are often 

 characteristic of particular species, and that the sounds are pro- 

 duced most frequently and with greater intensity during the 

 breeding season than at any other time. While useful to all 

 Fishes that possess them, vocal organs are, no doubt, specially 

 serviceable to those Fishes which, from the nature of their habitat, 

 can make but little use of their eyes ; and this fact may perhaps 

 explain the prevalence of such organs in the Siluridae, which 

 are frequently bottom- or ground-feeding Fishes, and often live in 

 muddy waters. 



The sounds emitted by Fishes may also, in some instances at 

 least, be warning sounds. Many of the sound-producing Fishes 

 are provided with exceptionally strong spines either in connexion 

 with the median and paired fins, as in many Siluridae, or on the 

 general surface of the body, as in Diodon hystrix. Such spines 

 are very effective weapons for offensive or defensive purposes, 

 and are capable of inflicting very severe w r ounds. The natural 

 enemies of these Fishes learn by experience or instinct to 



