CHAPTER XX 



STRUCTURE AND DEVELOPMENT OF 

 CHORDATA VERTEBRATA 



Sub-Phylum Craniata. Classes : — Cyclostomata, 

 Pisces, Amphibia, Reptilia, Aves, Mammalia 



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. 



General Characters 



Vertebrates are coelomate Metazoa, with a segmental 

 arrangement of parts. The central nervous system lies in the 

 dorsal median line, and is tubular in its origin. A skeletal rod 

 or notochord, 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 persist in adult life, are always developed, but above 

 Amphibians they are restricted to embryonic life, are not 



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