Epilogue: What Comes of Studying Vertebrates 791 



From Primitive Chordate to Man 



If the history of a half-billion or more years be condensed into a 

 few sentences, the story of the vertebrates might go somewhat as 

 follows: In early Paleozoic time there must have existed some very 

 small, free-swimming, aquatic animals whose form probably resembled 

 that of quite young amphibian tadpoles. Devoid of paired appendages, 

 they swam by undulatory motions of the body and such tail as they 

 may have had. Dorsal to the digestive tube was a median supporting 

 rod, a notochord. They breathed by gills situated in paired pharyngeal 

 clefts. The central nervous organ was a dorsal hollow cord slightly 

 enlarged at its cephalic end. 



The origin of such purely hypothetic primitive chordates is 

 problematic. Whether derived from nemerteans (Hubrecht, 1883), or 

 from annelid worms (Dohrn and Semper, 1875; Delsman, 1913), or 

 from arthropods (Gaskell, 1908; W. Patten, 1912) is a question about 

 which much has been said and written and nothing proved. Bateson 

 (1886) regarded Balanoglossus as a very direct and little-altered de- 

 scendant of the primitive chordates. According to Kowalewsky (1868) 

 and W. K. Brooks (1893), the tunicate larva and the free-swimming 

 tunicate Appeudicularia are close to the primitive chordates. It is 

 generally agreed that the modern protochordates, with some doubt in 

 the case of the Hemichorda, are in one way or another closely related 

 to the primitive chordates. Amphioxus is probably a direct but rather 

 specialized descendant of early chordates. In its possession of some 

 definitely fishlike characteristics, it is probably an example of parallel- 

 ism in evolution. The Cyclostomata, commonly classified as verte- 

 brates, show, especially in the larval stage, strong points of resemblance 

 to Amphioxus (Figs. 315, 316) and in some respects are more fishlike 

 than Amphioxus. 



The typical fishes, Pisces, must have diverged at an early time 

 from those chordate lines which lead down to modern protochordates 

 and to cyclostomes. The earliest known definitely piscine chordates 

 were sharklike. It is a widely, but not unanimously, accepted view thai 

 elasmobranch fishes were the ancestors of all modern fishes and of all 

 other vertebrates. Descendants of early sharks became diversified 

 along several lines, producing the crossopterygians and fishes of the 

 several "ganoid" types, the lungfishes (Dipnoi), and, along a line 

 achieving the maximum of piscine specialization, the now dominant 

 Teleostei. 



At this point we encounter what is perhaps the most serious break 

 in the history of the vertebrates — the origin of amphibians. Whether 

 they are derived from Dipnoi or from early crossopterygians, pre- 



