LEPTOCARDII. 161 
Skeleton.—The skeleton, that in the preceding forms 
has been upon the outside, is now internal. In the sharks, 
sturgeons, etc., it is soft, and composed of cartilage, while 
in others it is of bone. The backbone is composed of a 
tow of bones, called vertebrae, that extend from the head 
to the tail, known collectively as the vertebral column. 
In the upper portion of each vertebra, except those in the 
tail, will be found a hole or arch (Fig. 189, CS), through 
which extends a long, white cord, connected with the brain, 
called the cerebro-spinal cord, that is thus protected from 
injury. The various bones of the head, that are extensions 
of the backbone, form a box or covering for the brain, 
known as the cranium or skull. The limbs never exceed 
two pairs, and are fundamentally the same in all Verte- 
brates, merely being adapted to the habits of the animal 
in flying, leaping, swimming, digging, clinging, or walking. 
These are features that characterize all Vertebrates, except 
the lowest forms. The peculiarities of structure that dis- 
tinguish the different divisions will be treated under the 
following heads, that represent the different classes of the 
backboned animals: 1. The lancelet; 2. The lamprey; 3. 
The true fishes; 4. Amphibians; 5. Reptiles; 6. Birds ; 
and 7. Mamrnals. 
Class I.— LEPTOCARDII. 
The Lancelet (4mphioxus).—This worm-like creature 
(Fig. 190) is the lowest animal in which we find a notochord 
in the adult. The body is lance-shaped, having no skele- 
ton, brain, cranium, or paired fins ; yet it has a backbone, 
represented by a cartilaginous string, called the notochord 
(Fig. 190, 7), with a nerve-cord, zv, over it. The heart 
is long and simple, and the blood colorless. The mouth, 
m, is oval, surrounded by delicate tentacles ; and the eyes 
are mere specks, ¢. The young pass through a metamor- 
phosis, being at first oval, ciliated bodies; later, resem- 
