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 wounds. The natural 
enemies of these Fishes learn by experience or instinct to 
