no STRUCTURE OF THE VERTEBRATES 



In the chondrostean fish (the sturgeon and its relatives) the 

 denticles are fused into plates which are relatively large, cover- 

 ing the head with a close-set group of dermal bones, and the 

 body with bony plates. In this and the higher groups the ecto- 

 dermal part of the dermal plates is lost. The head plates and 

 the body plates are continuous in these fish, and the former are 

 not fused to the underlying chondrocranium. The teeth in the 

 bony fish and the land vertebrates have become well differenti- 

 ated from the dermal plates, and retain the enamel covering. 



In the Holostean fish (page 40) there is a more marked dif- 

 ference between the head and body plates. The former fuse to 

 the skull bones which developed from the chondrocranium, form- 

 ing a definite skull derived from both dermal and replacement 

 bone. This same relationship is carried through all the higher 

 classes of vertebrates. The Teleostei (page 42) are essentially 

 like the Holostei, though the dermal scales lose the heavy bony 

 characteristics of the earlier fish, and overlap as flexible scales. 



The primitive Crossopterygii (page 44), which gave rise to 

 the Amphibia, had large heavy scales practically continuous 

 with the head plates. Naturally this primitive relationship was 

 carried over to the Stegocephalia. These extinct amphibia had a 

 completely ossified skull, a ventral group of dermal plates, and 

 a dorsal covering. The recent amphibia have lost all signs of 

 body plates, except for very minute ones buried in the skin of 

 the Apoda (page 50). 



The amphibian line which branched toward the reptilian line 

 of ancestry retained the body plates. These were a protective 

 mechanism and were of selective value in the constant struggle 

 for food. Practically all the early reptiles had these dermal 

 plates, which, by analogy with the recent forms, we presume 

 were embedded in the skin and covered by the ectodermal layer. 

 Two lines of evolution, as far as the dermal structures are con- 

 cerned, are apparent. The large, sluggish reptiles tended to keep 

 the plates, for in these animals they are of protective value; 

 the smaller, more active animals tended to lose them, for the 

 race in life is to the swift or well protected. And the student 

 should remember that racial survival depends upon individuals 

 remaining alive. Among the recent reptiles the snakes and liz- 

 ards lack the plates, the other three orders retain them. 



