GENERAL OUTLINES OF ICHTHYOLOGY. V 



Duly to apprehend the various plans on which fish are 

 constructed requires a thorough acquaintance with piscine 

 organization in all its forms ; and even a condensed trea- 

 tise on such a subject would necessarily occupy a volume. 

 In the present outline, therefore, the general character- 

 istics and most remarkable features only are brought 

 before the reader, his attention being more particularly 

 directed to those organs upon the modifications of which 

 the classification of the various families and species, espe- 

 cially of fresh- water fish, is based. 



Fishes are, in scientific phraseology, " Oviparous Verte- 

 brata that is, vertebrate animals bringing forth eggs 

 with a double circulation, and respiring through the me- 

 dium of water." 



In common with other animals of the same division, 

 they possess a continuous longitudinal nervous axis, com-? 

 monly 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 connexion 

 with the muscles governing the movements of the fish. 

 The vertebrae, or joints of the spine, forming the back- 

 bone, in which this nervous axis lies, are in many fishes 

 wholly gristly, or cartilaginous, but in others more or 

 less bony. 



The term ( spinal column' includes both the nervous 

 axis, or spinal marrow, and its envelope. 



