STRUCTURE OF FISHES. II 



than the cerebral lobes cf the brain, as they are in the genera Polypterus and Lepidosiren. In the 

 Blind Fish (A mblyopsis) the optic lobes are exceedingly small. In several fishes, such as the Sturgeon, 

 the optic lobes are almost completely united into one mass, but even where they are most widely 

 separated, as in the Perch and the Herring, they are connected by a transverse band which passes in 

 front of the third ventricle. The cerebral hemispheres often have a pinkish appearance, and the 

 nervous matter is fissured and sometimes nodulated, but never approaches the convolute character 

 seen in the higher mammals. These nervous masses are large, smooth, and elongated in many fishes, 

 and in the Sharks become blended together ; but in the bony fishes the cerebrum is proportionately 

 small, especiallv in the Herring. It is also small in the Perch and Bream, but is relatively 

 largest in the Ganoid fishes, which have hemispheres exceptionally large. The cerebral lobes are usually 

 solid, but sometimes contain a lateral ventricle. The olfactory lobes are two distinct masses of grey 

 nervous matter, which are never united by a transverse band, and may be in contact with each other 

 in front of the cerebrum or widely separated. The true olfactory nerve consists of a group of distinct 

 fibres, where it is given off from the olfactory lobe of the brain. The cerebral hemispheres of fishes 

 correspond to that portion of the brain in mammals known as the corpora striata. 



The relative size of the brain to the body may be gathered from the fact that in a Carp 

 weighing 11,280 grains the brain weighed fourteen grains. As with higher animals, the brain 

 acquires its full size before the fish has attained its full growth, and hence is relatively smaller 

 in old fishes than in young ones. The great development of the medulla oblongata in fishes has a 

 direct relation to the large size of the respiratory organs, or gills. The development of the 

 cerebellum, so remarkably seen in the Sharks and Rays, is connected with active locomotion. 

 Sir Ricli. Owen observes that there is a distinct relation between the form of the brain and' 

 the habits of the fish, but all fishes of the same habit have not the same types of brain. " Thus- 

 the Shark and Pike are ferocious and predatory, the Angler and Skate are crafty, the Sword-fish and 

 Stickleback love fighting, and the Barbel and Carp are timid, peaceful browsers. If the cerebral 

 hemispheres of the Shark and Pike are compared, these parts of the brain differ more in shape, size, 

 and structure than in any other fishes, though they are equally sanguinary, equally insatiable, both 

 unsociable, and are tyrants, one of the sea and the other of the lake. The cerebrum of the Pike is 

 smaller than the cerebellum ; in the Shark it is larger than all the rest of the brain. In the Pike 

 the two lobes are distinct, and united only by a narrow transverse band, but in the Shark they are 

 blended into one large globular mass. In the Pike the cerebral lobes are narrow, but in the Carp it 

 feeds upon they are broad, and in the fighting Stickleback the cerebral hemispheres are longer and 

 narrower than in the cowardly Gudgeon." 



The organ of smell in fishes has no connection with the mouth, and is in no way connected witta 

 respiration, as in higher animals. In the Lamprey and Hag (Myxine) it is single, but in all other fishes, 

 there are two olfactory organs. In osseous fishes these organs are placed at the sides of the snout. 

 The Wrasses have a single opening for each nose sac, but in many fishes there are two, and then the 

 anterior one is closed by a valve or circular muscle, and the posterior one is open. In the Sharks 

 and Skates the nasal cavities are on the tinder side of the snout, and here the single wide opening is 

 defended by a valve. The organs of sight in fishes are marked by a few peculiarities ; thus, there is 

 no lachrymal gland, the eyes apparently being sufficiently moistened by contact with the water. In 

 the Hag and some other fishes the eye is a mere speck coated with dark pigment, but, as a rule, in 

 osseous fishes the eyes are large, and are especially conspicuous in the Sun-fish. The crystalline lena 

 is large and firm ; the fibres which form it usually converge to two poles, like the meridians on a globe, 

 but in the Salmon tribe and Sharks they converge to a line on each side. It was found by Sir David 

 Brewster that the fibres of the crystalline lens in the Cod are locked together by teeth, like those on 

 cog wheels. He calculated that in the eyes of a Cod there are five millions of fibres, on which there 

 are sixty-two thousand five hundred millions of these teeth, and yet in the living animal the organ is 

 perfectly transparent. The pupil of the eye is usually round, but in many Sharks it is elliptical, and 

 in the genus Galeus it is four-sided. In the Skates and flat fish a remarkable fringed process is 

 connected with the upper margin of the pupil, and is capable of being let down and drawn up like a 

 curtain, to regulate the quantity of light admitted to the eye. This would seem an arrangement to 

 supplement the feeble contraction of the iris in fishes. In the Sharks and San-fish the eye is contained 



