CLASSIFICATION AND ORIGIN OF FISHES 63 



lower jaws, teeth, and paired fins. The present-day lampreys and hag- 

 fishes, though highly specialized, are probably modern relics of these 

 early types. 



The sharks appeared soon after these early forms, and although their 

 skeletons were of soft cartilage, they had a definite and characteristic 

 skeleton. They had upper and lower jaws developed from the front gill- 

 arch. The primitive placoid scale appeared, the enlarged scales on the 

 margin of the jaw developed into teeth, and the paired fins appeared. 

 These early sharks were the first true fishes. At one time, ages ago, they 

 comprised all the fishes in the world, and many still remain in the sea 

 today. 



The modern Osteichthyes, to which true fresh-water fishes belong, 

 undoubtedly developed from some sharklike ancestor. They first ap- 

 peared as a series of primitive fishes with cartilaginous skeletons cov- 

 ered by bony plates and scales. A few of this primitive type still remain 

 as sturgeons, paddlefish, gars, and dogfish. 



One group of these primitive fishes, now virtually extinct, learned to 

 breathe air by means of the swim bladder. By means of their paired fins 

 they crawled out of the water on to the banks and found a world where 

 there was no competition from other vertebrates. Some of these fishes 

 never returned to the water but became the ancestors of the land verte- 

 brates. Lungs probably developed from swim bladders, and the paired 

 fins became fore and hind legs. 



Another group of these primitive bony fishes became the ancestors 

 of the modern bony fishes. They incorporated into the skull the bony 

 plates covering the head, lost at least one of their gill-shts, replaced 

 most of the soft cartilage in their skeletons with hard bone, and formed 

 bony spines and rays to support the fins. Most of the modern fishes have 

 departed as widely from the form and structure of their primitive ances- 

 tors as have the land vertebrates descended from the same ancestors. 



To aid in the classification of the fishes described in this book, the 

 following taxonomic keys to the families and to the species are included. 

 If the family is unknown, it may be found by using the family key, 

 assuming that the person using the key has studied the pages describing 

 the structure of the fish. To use the key, select the part of each couplet 

 that agrees with the structure of the fish, and continue to the couplet 

 indicated by the number at the right, until the family is reached. Then 

 find the page indicated for the family and follow through the key to the 

 species. In those families represented by only one species no key to the 

 species is necessarj\ 



