OUTLINES OF ICHTHYOLOGY. 151 



ward, end of the nervous axis the brain is formed, 

 with which the nerves of smell, sight, taste, and 

 hearing are connected. Immediately under the 

 spinal column, and divided from each other by 

 their membranes, are situated the organs of respi- 

 ration, circulation, digestion, excretion, and also, 

 when it exists, the air- or swim-bladder. This 

 type of structure is common to all vertebrate 

 animals ; but the peculiar attribute of Fishes is, 

 that in them the function of respiration, or oxygen- 

 ation of the blood, is performed through the me- 

 dium of water, not merely temporarily, but during 

 the continuance of life. 



Duly to apprehend the various * plans ' on which 

 fish are constructed requires a thorough acquain- 

 tance with piscine organization in all its forms. 

 Even a condensed treatise on such a subject would 

 necessarily occupy a volume. In the present out- 

 line, therefore, I shall only attempt to bring before 

 the reader the general characteristics and most 

 remarkable features, his attention being more par- 

 ticularly directed to those organs upon the modifi- 

 tions of which the classification of the various 

 families and species, especially of fresh-water fish, 

 is based. 



The first and most important function of fish 

 and all other animals is breathing. For the pur- 

 pose of breathing fish are provided on each side of 



