ASCIDIANS AND VERTEBRATES. 1 53 



The early stages of the Ascidia possess the beginnings of the 

 spinal marrow and the spinal column (chorda dorsalis) 

 lying beneath it, which are the two most essential and most 

 characteristic organs of the vertebrate animaL Accordingly, 

 of all invertebrate animals known to us, the Tunicates are 

 vdthout doubt the nearest blood relations of the Vertebrates, 

 I and must be considered as the nearest relations of those 

 ^"v. Worms out of which the vertebrate tribe has developed. 

 (Compare Plates XII. and XIII.) 



While thus different branches of the Coelomatous group 

 of the Worms furnish us with several genealogical links 

 leading to the four higher tribes of animals, and give us im- 

 portant phylogenetic indications of their origin, the lower 

 group of Accelomi, on the other hand, show close relation- 

 ships to the Zoophytes, and to the Primaeval animals. The 

 great phylogenetic interest of the Worm tribe rests upon this 

 peculiar intermediate position. 



