FOURTH BRANCH OF ZOOLOGY. 



ICHTHYOLOGY. (Gr. **0a s , ichthus, a fish ; Myog, Zogos, a 

 discourse.) 



Class, Pisces. FISHES. COLD-BLOODED VERTEBRATES. 



SECTION VII. 



WE come now to that part of Zoology which treats of Fishes, 

 their structure and form, their habits and uses, and their classi 

 fication. The Fishes, as a class, possess a greater number of 

 species than any other of the primary divisions of the Vertebrates ; 

 and, indeed, the species not improbably exceed in number those of 

 the Mammals, Birds, and Reptiles taken together. Our limits 

 will therefore allow us to do nothing more than present a gen- 

 eral view of this part of the Animal Kingdom, followed by suc 

 cinct notices of the orders and families. 



The most prominent characteristics of Fishes are (1) that they 

 generally have cold red blood; (2) they breathe by gills instead 

 of lungs; (3) they have a two-chambered heart; (4) they use 

 fins as organs of progression ; (5) they have the skin naked or 

 covered with scales of varied structure; (6) they are almost 

 incredibly prolific. 



The blood, generally cold, assumes the temperature of the 

 surrounding element. It should be stated, however, that in some 

 of the swift Oceanic Fishes of the Mackerel family, such as the 

 Tunny and the Bonito, the blood is found to be 10o higher than 

 that of the surface of the sea, even within the tropics. The 

 blood-disks are sometimes circular, and sometimes oval ; they 

 are larger than those of the mammalia and birds, smaller than 

 those of reptiles, especially the amphibia. The gills consist of 

 bony or cartilaginous spines, usually placed parallel with each 



