150 OUTLINES OF ICHTHYOLOGY. 



vertebrata vertebrate animals bringing forth eggs 

 with a double circulation, and respiring through 

 the medium of water." Of such vertebrata, that 

 is, animals possessing a backbone they form the 

 fourth or lowest class in the great systematic divi- 

 sion of the Animal Kingdom ; and from the fact 

 that the salt waters alone occupy more than seven- 

 tenths of the globe's entire surface, and are in all 

 probability inhabited, stratum super stratum, as far 

 as, or even further down than the rays of the sun 

 can penetrate, it will readily be conceived that fish 

 are not only the most numerous of all vertebrate 

 animals, but must infinitely outnumber the whole 

 of the other three classes. 



In common with other animals of the same divi- 

 sion fish possess a continuous longitudinal ner- 

 vous axis, commonly called the " spinal marrow," 

 situated in the centre of the spine, and composed 

 of four parallel columns, one pair of which receive 

 the nerves of sensation, coming from the surface 

 of the body, and the other pair form the roots of 

 the nerves of volition, or action, in connection with 

 the muscles governing the movements of the fish. 

 The vertebrce, or joints of the spine, forming the 

 backbone, in which this nervous axis lies, are in 

 many fishes wholly gristly, or cartilaginous, but in 

 others more or less bony. The term " spinal co- 

 lumn " includes both the nervous axis, or spinal 

 marrow, and its envelope. 



By an enlargement of the anterior, or more for- 



