GENERAL ZOOLOGY 



Chondrichthyes '' ^ 



Placodermi 

 (archaic armored fishes) 



(shark-hke fishes) 



Agnatha 

 (jawless fishes) 



Fig. 18.8. A simplified family tree of vertebrate groups. (From A. S. Romer, . 

 Vertebrates, copyright 1941 by University of Chicago Press, reprinted by permission.) 



(From A. S. Romer, Man and the 



higher forms the primitive axial skeleton is replaced by the vertebrae. The 

 great majority of vertebrates have paired appendages, either fins or limbs. 

 The pharynx is of moderate size and bears gill slits, which are permanent in 

 the lower forms but transitory in the vertebrates which develop lungs. The 

 vascular system is closed, with a well-developed heart and blood containing 

 red cells, or erythrocytes. Additional features of the vertebrates include 

 paired eyes and ears, cranial and spinal nerves, a coelom, and a single pair of 

 gonads. The relationships of some of these structures are represented in 

 Figure 18.7, a diagram of a generalized vertebrate. Separation of the Verte- 

 brata into gill-bearing forms with fins, the Pisces, and air-breathing forms 

 with limbs, the Tetrapoda, reflects the apparent evolution of these animals 

 from an aquatic to a terrestrial mode of life. 



The Pisces or Fishes. As the first type of vertebrates (Fig. 18.8) known 

 from the fossil record, the Pisces have had a long and diversified history. 



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