10 



NATURAL HISTORY. 



To the two latter groups the great multitude of living fishes belong, but in the earlier ages of the 

 earth's history Ganoid fishes were the prevailing types. Hence the older and less perfectly ossi- 

 fied division of fishes has been named by Dr. Giinther, Palseichthyes. 



The nervous system in fishes presents an unusual amount of variation. In the Skates there is a 

 slight enlargement of the spinal column in the region where the large nerves are given off to the pectoral 

 fins, and the same condition may be noticed in a less degree in the Sharks, but no corresponding enlarge- 

 ment of the spinal cord has been noticed in the Flying-fish or any other osseous fishes. In the Sturgeon 

 there is a slight enlargement of the spinal cord at the beginning of the caudal region ; in the Sun-fish 

 the spinal cord is said to be reduced to a short conical appendage to the brain ; and in the genera Tetrodon 

 and Diodon the spinal cord is exceedingly short and small, but it has not the ganglionic structure 

 seen in the Sun-fish. But in most fishes the spinal cord is as long as the neural canal. It is often 

 marked by longitudinal fissures along the ventral and dorsal sui-faces ; and in the Sturgeon there is a 

 less complete lateral groove dividing the spinal cord into dorsal and ventral columns. And in many 

 fishes, such as the Cod and Herring, six cords may be distinguished : two of which are dorsal, and 

 govern sensation ; two ventral, and govern motion ; and there are also two lateral regions. As the 

 spinal cord approaches the brain it enlarges. According to Sir Rich. Owen, fishes are especially 

 distinguished by having lobes which correspond to the great vagus nerve, or pneumogastric, as it is 

 usually called, extending into the fourth ventricle of the brain, which is a cavity at the beginning of 



the medulla oblongata, or part where the brain becomes 

 connected with the spinal cord. 



The brain has its parts always arranged one behind 

 the other in longitudinal succession, as among Amphibia 

 and Reptiles. There is a hindermost part, which is single, 

 called the cerebellum, which in most fishes is comparatively 

 small, but becomes large and marked with transverse folds 

 in the Sharks and Rays. Placed in front of this are two 

 more or less rounded or ovate masses of brain, called the 

 optic lobes. Farther still in front is the cerebrum, which 

 usually consists of two masses, which may be larger or 

 smaller than the optic lobes ; but among the Sharks and 

 Skates these masses of the brain are usually more or less 

 blended together. In many fishes there are, besides, large 

 olfactory lobes placed in front of the cerebrum, and from 

 these the nerves of smell are prolonged. The nerves ai-e 



given off from the brain, precisely as in other animals, and the spinal cord prolonged down the 

 vertebral column gives off nerves usually from between the vertebrae, though occasionally, as among 

 some Sharks, they pass through perforations in the bony arches which cover the spinal cord. 



In most osseous fishes the cerebellum is smooth and convex ; it is frequently hemispherical, as in 

 Amblyopsis, a genus of blind fish. In the Eel it is transversely elliptical ; in Lepidosteus it is longi- 

 tudinally elliptical ; it is oblong in the genus Diodon ; it is a depressed tongue-shaped body in the 

 Cod ; it is pyramidal in the Perch, and attains an immense development in the Sharks, where it 

 extends over the optic lobes, which is also the case in the genus Amblyopsis ; while in the Saw-fish 

 it extends forward so far as to rest upon the cerebrum. It is largest in the most active fishes, is very 

 small in the Lump-fish, is unsymmetrical in some of the flat fish, has a longitudinal groove in the 

 genus Diodon, and is transversely divided in the genus Lophius. The fishes in which it shows the 

 branching interior structure called the " arbor vitae," due to the grey matter being folded over the 

 white nervous matter, are Sharks and the Tunny. Another peculiarity of the Skate tribe, and found 

 in most of the allied fishes, is the development of large convoluted lobes at the sides of the medulla 

 oblongata, in the position where the fifth nerve is given off, a condition well seen in the Torpedo and 

 in the Chimara monstrosa. The optic lobes are usually the largest portion of the fish brain; they are 

 spheroidal. Prolonged downward from this region is that remarkable part of the brain called the 

 pituitary body, and upward the pineal gland is given off in front. There is a cavity in the optic 

 lobes, which is one of the ventricles. It is quite exceptional for the optic lobes to be smaller 



DIAGRAM OF BRAIN" OF CODFISH. 

 , Upper; B, Side; c. Under; D, Section. 



