VERTEBKATA. 



All of the forms so far studied or described are associated 

 together as a group or branch Vertebrata the name of 

 which implies that they all possess a " back-bone " composed 

 of separate bones or vertebrae. This one character of itself 

 would hardly warrant this grouping, especially since some 

 forms have the vertebrae but feebly developed, while in other 

 features they are closely similar to those with a well-devel- 

 oped back-bone. This presence of vertebrae is closely asso- 

 ciated (correlated) with other features of equal or even of 

 more importance, and it is this totality of similarity that 

 justifies the group. 



All vertebrates have an inner supporting skeleton, and a 

 few forms, like the turtles, have in addition an external 

 skeleton derived from the skin. The internal skeleton, for 

 convenience of treatment, may be divided into one portion 

 lying in the axis of the body, and a second portion pertain- 

 ing to the limbs and appendages. Besides these there is a 

 third part, the visceral skeleton, developed in connection 

 with the gills. 



The axial skeleton consists of the vertebral column (back- 

 bone), the skull, and the ribs. In all vertebrates, at least 

 in the young stages, a solid rod of cartilage runs through 

 the body between the central nervous system and the 

 alimentary canal. In front it terminates near the middle 

 of the brain; behind it runs to the end of the body. This 

 rod is the notochord. In the higher vertebrates it dis- 

 appears long before the animal becomes adult; but in the 



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