250 AN INTRODUCTION TO ZOOLOGY. 



whicli has two lateral fin. ridges exactly like its 

 median one, only that they do not extend all the way 

 to the tail. 



The differences between fishes and the other Verte- 

 brates may be summarized by saying that they retain 

 a primitive elongated form and metameric symmetry 

 which reminds us of articulated animals, while the 

 higher Vertebrates have many of their parts doubled 

 and folded upon one another, so as quite to obliterate 

 their original arrangement. For instance, the brain of 

 fishes consists of parts placed one behind another in 

 a row ; but in mammalia, and most of all in the human 

 subject, these parts are doubled over one another in a 

 kind of zigzag — an arrangement which results from 

 the fact that in the mammal the cerebral hemispheres 

 are much more developed than the other parts, and 

 have to make room for themselves by overlapping 

 them. 



Fjshes. 



Classification of Fishes.— Fishes are divided 

 into the orders described below. Excluding the 

 Amphioxus, which has already been spoken of, the 

 Oyclostomi or jlfai-sipobrancJdi, comprising the Hag- 

 fishes and Lampreys, are the lowest order of fishes. 

 They have round suctorial mouths, as is indicated by 

 their first name, and they have six or seven pairs of 

 pocket-like gills. They live in mud, and hold on by 

 their suctorial mouths to stones, or to whatever they 



