CHAPTER XX. 



STRUCTURE AND DEVELOPMENT OF VERTEBRATA. 



SINCE the time of Aristotle over two thousand years ago 

 the distinction between backboned and backboneless 

 animals must have been more or less evident to all who, 

 with any precision, thought of the forms of animal life. 



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

 separation was drawn with adequate 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. 



From what has been said in regard to these three types, 

 it will be plain that the apparent apartness of the Vertebrata 

 was thus annulled. 



GENERAL CHARACTERS. Vertebrates are ccelomate 

 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 is formed 

 as an outgrowth along the dorsal median line of the primitive 

 gut, but though this is always present in the embryo at least, 

 it tends to be replaced by a mesodermic axial skeleton the 

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

 sist in adult life, are always developed, but gill-lamellce do not 

 occur above Amphibians. The heart is ventral The eye 

 begins to develop as an outgrowth from the brain. 



