CHAPTER 30 



THE TETRAPODA: AMPHIBIA, REPTILES, 

 AND BIRDS 



The full potentialities of vertebrate development were not realized 

 until the vertebrates emerged from the water to become terrestrial 

 dwellers. Once this step has been taken, new and unexploited habits, 

 habitats, and ways of life were available. The emergence of the verte- 

 brates from water to land must have been a very gradual process in- 

 volving countless unsuccessful experiments. Probably the first verte- 

 brate to free itself at least partially from the water was some amphibian- 

 like animal which was forced to live only in very moist areas. 



The modern fish give us only vague hints of the pathway followed 

 by the emerging vertebrates. The mudskipping goby, Periophthalmus, 

 is a strange little fish which emerges from water to sun itself on the 

 roots of mangrove trees. Its pectoral fins are modified into limblike 

 structures by which it moves about. By means of this highly modified 

 pectoral fin, the goby can raise the forepart of its body and drag itself 

 about in its search for insects ; however, the goby has no other adapta- 

 tions to land life and thus remains permanently fixed to its aquatic 

 existence. The sargassum fish, Pterophryne, has progressed a bit far- 

 ther than the goby inasmuch as its pelvic fins also are somewhat foot- 

 like. Neither of these has any adequate method for respiring on land ; 

 thus they do not show us the path followed by the first emerging forms. 



The modern lungfish with its modified swim bladder for respira- 

 tion would seem to give another clue as to the possible development of 

 structures necessary for a terrestrial existence. During periods of 

 drought, the lungfish forms a cocoon about itself and remains dormant. 

 Unfortunately, however, the lungfish has failed to develop adequate 

 locomotor structures for moving about. Their paired appendages are 

 little more than flexible, threadlike structures, incapable of supporting 

 the body ; thus the lungfish also fails to meet the stringent requirements 

 of the progenitor of the amphibians. 



592 



