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 produced 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 {365}associate particular sounds with the possession of dangerous spines, and warned by the sounds, they refrain from attacking the owner of the spines, to the mutual advantage of both. [Illustration: FIG. 209.—An Electric Ray (_Torpedo_) dissected to show its electric organs. On the left the nerves supplying the organ are dissected out. The prismatic areas on the surface of the organ indicate the vertical columns of electric plates, of which there may be 500,000 in each organ. The dorsal surface of the brain is exposed. _br_, Gills; _f_, spiracle; _o_, eye; _o.e_, electric organs; _t_, mucus canals; _tr_, tri-geminal nerve; _tr′_, its electric branch; _v_, vagus; _I_, fore-brain; _II_, mid-brain; _III_, cerebellum; _IV_, electric lobe of the medulla oblongata. (From Parker and Haswell, after Gegenbaur.)]