Phyhmi Chordata 567 



The Classification of the Subphylum Vertebrata. — There are 

 seven living classes of vertebrates which are as follows : 



Superclass Pisces 



Class I. Agnatha. Fish with jawless mouths, no paired appendages. Ex- 

 amples : ostracoderms ; lamprey eels, Petromyson ; and the hagfishes, 

 Myxine. 



Class II. Placodermi. Extinct, jawed vertebrates. 



Class III. Chondrichthyes. Fish possessing cartilaginous skeletons and 

 placoid scales. Examples : dogfish, Squalus ; sharks, rays, and 

 chimaeras. 



Class IV. Osteichthyes. Fish typically possessing a skeleton of bone. 

 Scales may be cycloid, ctenoid, ganoid or lacking. Examples : perch, 

 Perca; carp, Cyprinus. 



Superclass Tetrapoda 



Class I. Amphibia. Animals possessing a moist skin without scales, usu- 

 ally with an aquatic larva and terrestrial adult. Examples : frog, Rana ; 

 toad, Bufo; salamanders, Ambystoma. 



Class II. Reptilia. Animals possessing scaly skins and with eggs having 

 well-developed membranes. Examples : turtles, snakes, lizards, croco- 

 diles, and alligators. 



Class III. Aves. Animals possessing feathers, with eggs having em- 

 bryonic membranes, warm-blooded. Example: birds. 



Class IV. Mammalia. Animals having the body covered with hair, warm- 

 blooded, nourish young by means of mammary glands. Example : 

 mammals. 



The Origin and Development of the Pisces 



The first vertebrate of which there is any fossil record is the fish- 

 Hke, armored ostracoderm (Fig. 193,c). The earliest of these date 

 from the fossil beds of the Ordovician period which occurred some 

 440,000,000 or so years ago and lasted for about 60,000,000 years. The 

 ostracoderms attained their greatest abundance during the Devonian 

 period about 350,000,000 years ago. In appearance they were shaped 

 vaguely like the horseshoe crab, possessing a head shield with eyes 

 located on top and a single dorsal nasal opening. Ventrally, they had a 

 jawless, rounded mouth. The area under the shield was largely occu- 

 pied by a gill chamber, and it is probable that they attained their food 



