THE PHYLA HEMICHORDATA AND CHORDATA 



rasping their way into the flesh to such an extent that they can ahnost be 

 regarded as internal parasites. Both lampreys and hags are represented by 

 only a few genera, distributed in the ocean and in fresh water. 



The Chondrichthyes. This class includes sharks, skates, and rays (Fig. 

 18.10). In contrast to the Osteichthyes, these fishes have a cartilaginous 

 endoskeleton. This was long regarded as a very primitive feature in the 

 evolution of vertebrates, but it now appears that the earliest known 

 Chondrichthyes had bony skeletons. Thus, the existing forms must represent 

 a line of descent in which the skeleton has degenerated from an earlier bony 

 type to its present cartilaginous condition. Obvious specializations of the 

 Chondrichthyes beyond those of the Agnatha include biting jaws, teeth, and 

 paired fins. The jaws appear to have developed from certain skeletal bars of 

 the gill region, and the teeth from scales similar to those in the skin of a 

 modern shark. Sharks' teeth and remnants of the skeletal elements of the 

 skin, well developed in many of the early forms, are much more abundant as 

 fossils than are other remains of these animals. The fossils in some of the 

 Devonian deposits, however, have enabled palaeontologists to make fairly 

 complete restorations of types that were primitive, yet specialized in their 

 own ways. The primitive sharks apparently originated in fresh water, from 

 some ostracoderm stock, and subsequently migrated to the ocean. They 

 then disappeared from fresh water. Ever since this invasion of the ocean, 

 sharks of many kinds have been among the largest of the marine fishes. They 

 are now familiar predators of the surface waters in the ocean, where they 

 attack a wide range of prey. The skates and rays, with their specializations 

 for life on the bottom of the ocean, represent divergent types. They com- 

 monly live in shallow waters, feeding upon mollusks, crustaceans, and other 

 sluggish bottom-dwelling animals. The sting ray, with a venomous spine in 

 its tail, and the torpedo ray, with a powerful electric organ, are highly 

 specialized representatives of this group. 



The Osteichthyes. In contrast to the existing Agnatha and Chondrichthyes, 

 with cartilaginous skeletons, most members of the class Osteichthyes (Figs. 

 18.11, 18.12) have bony skeletons; however, the bones of this skeleton are not 

 so firmly articulated as those of land vertebrates. As we have seen, bone did 

 not originate with the Osteichthyes; it was present in the skeletons of early 

 Agnatha and Chondrichthyes. The first representatives of the Osteichthyes 

 appear as fossils in the middle Devonian, soon after the earliest sharks. 

 These early Osteichthyes are separable into two groups, the ray-finned and the 

 lobe-finned fishes. From the ray-finned group have descended the modern ray- 

 finned fishes; from the early lobe fins have descended both the land verte- 

 brates (Tetrapoda) and the existing lungfishes, called Dipnoi. 



Surprising as it may seem, lungs appear to have been present in many, if 

 not all, of these early fishes. Perhaps lungs arose as an adaptation favoring 

 survival in stagnant pools which may have been formed recurrently in the 

 watercourses, under the climatic conditions of the Devonian. After making a 

 beginning of air breathing, one line of early fishes, the lobe-finned fishes, 



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