CHAPTER XX 
STRUCTURE AND DEVELOPMENT OF 
VERTEBRATA 
Tue 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 ot 
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 celomate Metazoa,with a segmental arrange- 
ment of parts. The central nervous system lies in the dorsal 
median line, and ts 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 gili-slits, which may or may not per- 
sist in adult life, are always developed, but above Amphibians 
they are restricted to embryonic life, ave not directly functional, 
and have no associated gill-lamelle. The heartis ventral, The 
eve begins its development as an outgrowth from the brain, 
