LEPTOCARDIL 151 



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 

 row 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, C S), 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 : i. The lancelet ; 2. The lamprey; 3. 

 The true fishes ; 4. Amphibians ; 5. Reptiles ; 6. Birds ; 

 and 7. Mammals. 



Class I. LEPTOCARDIL 



The Lancelet (Amphioxus). 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 notochcrd 

 (Fig. 190, r), with a nerve-cord, nv, 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, e. The young pass through a metamor- 

 phosis, being at first oval, ciliated bodies ; later, resem- 



