CHAPTER XX. 



STRUCTURE AND DEVELOPMENT OF 

 VERTEBRATA. 



The distinction between higher and lower animals, between 

 the backboned and the backboneless, was to some extent 

 recognised by Aristotle over two thousand years ago, and 

 was probably always more or less evident to any who cared 

 to look with precision at the forms of animal life. 



Yet it was not till about a century ago that the line of separa- 

 tion was drawn with firmness. This Lamarck did in 1797. 



But the doctrine of descent — the idea of organic evolu- 

 tion—with which Darwin impressed the thoughtful in 1859, 

 suggested inquiry into the apparently abrupt apartness of 

 the group of Vertebrates. 



The inquiry bore fruit in 1866, when the Russian 

 naturalist, Kowalevsky, worked out the development of the 

 Vertebrate characteristics of Amphioxus, correlated this 

 with the development of Ascidians, and discovered the 

 pharyngeal gill-slits of Balanoglossus. Thus the apparent 

 apartness of the Vertebrata was annulled. 



General Characters. 



Vertebrates are ccelomate Metazoa, with a segmental arrange- 

 7nent of parts. The central nervous system lies in the dorsal 

 median line, and is tubular in its origin. A skeletal rod or noto- 

 chord, formed as an outgrowth along the dorsal median line of 

 the primitive gut, is always present in the embryo at least, but 

 tends to be replaced by a mesodermic axial segmented skeleton — 

 the backbone. Pharyngeal gill-slits, which may or may not per- 

 sist in adult life, are always developed, but above Amphibians 

 they are restricted to embryonic life, are not directly functional, 

 and have no associated gill-lamella;. The heart is ventral. The 

 eye begins its development as an outgrowth from the brain. 



