SORTING OF ANIMALS 7 
“The Notochord as a structure, precedes the formation of the 
spinal column in vertebrates. The spinal column of vertebrates is 
formed to protect the spinal cord. This protection is, however, an 
afterthought, so to speak, of the vertebrate structure; the lowest of 
all vertebrates is quite without it, and in the lower groups of fishes 
we may trace various steps of its formation. But in these cases 
where the spinal column is absent or incomplete, there is a large and 
well-developed notochord; and in the embryo of higher vertebrates, 
when the spinal column has not yet begun to be formed, the noto- 
chord is equally a conspicuous feature. It runs from the region 
known as the midbrain, to the end of the tail, and lies throughout 
just beneath the spinal cord. Whatever its original use in the animal 
body may have been, it undoubtedly acts now as a support to the 
spinal cord, and indeed to the whole body. Bones do not exist either 
in the lower vertebrate, or in the early embryo. In the latter they 
are formed by degrees. The spinal cord and the notochord each 
begin to be surrounded by rings of cartilage or gristle, which by 
degrees is changed into bone. The rings surrounding the notochord, 
however, gradually encroach upon it and obliterate it. The place 
where it has been becomes the centrum, or most solid part of each 
vertebra. The notochord at first is continuous, and has no division 
into successive parts; but when the bony spinal column is developed, 
it consists of a series of successive vertebre. Each of them is made 
up of several parts, which by degrees become consolidated into the 
vertebre.” 
11. Fishes.—These are the lowest forms of backbone ani- 
mals. All fishes have gills for breathing, both the lowest 
fish order and the true fishes. The true fishes have scales, 
limbs and teeth. In some, such 
as sharks, rays and dogfishes, 
the skeleton is gristle and does 
not transform into bone. In 
nearly all of the familiar fishes 
the skeleton develops into bone. 
The kinds of fishes are legion 
HEAD OF TROUT, SHOWING GILLS and the shapes, colors, habits 
and character of many varieties. 
The truth about the matter is this: All the higher forms of ani- 
mals have gills for a time. In the highest vertebrates, such as rep- 
tiles, birds and mammals, the gills are never put to use. They exist 
in the early stages of embryo life, but afterwards disappear, other 
structures taking their place. 
12. Toads and frogs.—Between the fishes and the reptiles 
