CHAPTER XX. 



STRUCTURE AND DEVELOPMENT OF 

 VERTEBRATA. 



THE obvious distinction between higher and lower animals, 

 between the backboned and the backboneless, was to some 

 extent recognised by Aristotle over two thousand years ago. 



Yet it was not till 1797 that the line of separation was 

 drawn with firmness by Lamarck. 



But the doctrine of descent the idea of organic evolu- 

 tion which Darwin made current intellectual coin 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- 

 ment 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 bv a mesodermic axial segmented skeleton 

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

 sist in adult life, are alwavs 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. 



