290 SYSTEMATIC ZOOLOGY. 



encircled by a fringe of delicate filaments, appearing above 

 the surface. They are without any economic importance, 

 but their extremely simple structure makes them intensely 

 interesting to the naturalist. 



BRANCH III. VERTEBRATA. 



All of the forms associated together as a group or branch 

 Vertebrata receive this name, since they all possess a 

 'back-bone 7 composed of separate bones or vertebrae. This 

 one character of itself would hardly warrant this group- 

 ing, especially since some have the vertebrae but feebly 

 developed, while in other features they are closely similar 

 to those with a well-developed back-bone. This presence 

 of vertebrae is closely associated (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 jaws and 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 gelatinous tissue 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 no to chord. In the higher vertebrates it 

 disappears long before the animal becomes adult; but in 



