ASCIDIANS AND VERTEBRATES. 153 
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 
without doubt the nearest blood relations of the Vertebrates, 
and must be considered as the nearest relations of those 
Worms out of which the vertebrate tribe has developed. 
(Compare Plates XII. and XIII.) 
While thus different branches of the Ccelomatous 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 Primzeval animals. The 
great phylogenetic interest of the Worm tribe rests upon this 
peculiar intermediate position. 
