NATURAL HISTORY. 



in a hollow cartilaginous sheath, but usually the sheath is formed of two hemispherical cups, which are 

 sometimes cartilaginous and sometimes bony. There is often a good deal of fat between the outer 

 sclerotic layer and the more vital internal parts of the eye. In the fresh-water genus Anableps the 

 cornea is divided by an opaque horizontal line, on each side of which the iris is perforated by a pupil. 

 The muscles which move the eyes of fishes correspond with those of man, and are usually six in. 

 number. 



The organ of hearing is well developed in all fishes ; the membranous labyrinth in the Lamprey 

 has only two semicircular canals, and in the Myxine there is only one of these canals, but in all other 

 fishes there are thi*ee, as in higher animals; "bhey communicate with a vestibule, in which are contained 

 the bony plates called otolites; there are usually two of these flattened, somewhat oval organs, and one 

 is larger than the other. Bat in a good raariy fishes, svcli as the Plectognathi and Lophobranchiates, 

 the otolites are represented by calcareous dust. No fish possesses a cochlea or a true tympanic 

 .membrane, but sometimes there is a connection between the labyrinth of the ear and the air-bladder, 

 made by a chain of small bones. In the Loach the air-bladder is exceedingly small, extending under 

 only two vertebrae, and is united with the head in this way. The external ears in the Skate are on 

 the top of the head. 



Closely allied to the organs of sense must be classed the electric organs of fishes, though the electric 

 faculty is developed in very few genera. The best known of these are the Torpedo and the Electric Eel 

 (Gymnotus), though less powerful electric oi'gans exist in various species of Malapterurus, and are 

 said to exist in Trichiurus, Gymnarchus, and a species of Tetrodon. In the Torpedo there are two 

 electric organs; in the Electric Gymnotus there are two on each side of the body, where they occupy 

 almost the whole of the lower half of the trunk, and are arranged on the upper and lower sides of the 

 'body. The electric organs are relatively larger in the Gymnotus, but their electric power is less. 

 In the Malapterurus electricus the electric organ lies beneath the skin, and invests the whole body, 

 with the exception of the head and fins. The electric organ is here divided into minute lozenge- 

 , shaped cells, so that the fish is protected by an elective coat, but the shock from it is comparatively 

 :feeble. In the genus Mormyrus a gelatinous organ placed on each side of the tail was formerly 

 ibelieved to be electric. 



The teeth of fishes present a remarkable variety in their forms and numbers. Sir Rich. Owen 

 remarks that the Lophobranchii are toothless, as are the Sturgeon, the Paddle-fish, and Ammocetes, 



which is the larval form of the Lamprey, 

 requiring four yeai-s for its development. The 

 Myxine has a single-pointed tooth in the roof of 

 the mouth, and two serrated dental plates upon 

 the tongue. The Tench has one grinding-tooth 

 on the occiput, opposed to which are two jaws 

 in the pharynx below which bear teeth. In the 

 genus Chimsera the teeth in the maxillary bones 

 are confluent into two pairs, and there are two 

 teeth in the mandible, but in the Siluroids 

 and many other fishes the mouth is crowded 

 with teeth. A large number of fishes have 

 conical teeth; several of the Rays, like Mylio- 

 bates, have the teeth arranged like a tesse- 

 lated pavement. In the genus Citharinus the 

 teeth bifurcate at their extremities; in the genus 



Platax they divide into three points. Sometimes there are hemispherical teeth arranged like a 

 pavement, as in the Wrasse. The rarest position for teeth in a fish is upon the maxillary bone, 

 though they are developed 'there in Salmon and Herrings, and some Ganoids. In many fishes the 

 teeth' are blended with the jaw ; in Sharks their broad bases are usually attached by ligament ; in 

 the remarkable snout of the Saw-fish organs like teeth are implanted in sockets. Sometimes, as in 

 the "Wolf-fish (Anarrltichas), the front teeth are adapted for grasping shells, while the back teeth 

 are fitted for crushing them. In all fishes the teeth are shed and renewed many times during the 



JAWS OF MALE 



