14 PRIMITIVE ANIMALS 



nerves which issue from the nerve-cord between the 

 vertebrae. They also resemble the Appendiculata in 

 the possession of a coelomic body-cavity and a 

 separate blood vascular system. But the Chordates 

 differ from the Appendiculata in the fact that the 

 central nervous system or nerve-cord lies above 

 instead of below the alimentary canal, and the nerve- 

 cord itself is hollow, instead of being a double, solid 

 cord (Fig. 4). Although the Chordates resemble the 

 Appendiculata, and especially the Annelids, more than 

 they resemble any other Invertebrate phylum, there 

 are very grave difficulties in the way of deriving 

 them from the Appendiculata, a subject which will 

 be discussed in a future chapter. 



In the Chordate phylum, then, despite the fact 

 that in separate groups of the phylum some of the 

 most complete and convincing evolutionary series 

 have been worked out, the origin of the phylum 

 itself remains one of the most vexed questions of 

 zoology about which competent zoologists hold the 

 most diverse and contradictory opinions. 



Our short review of the animal kingdom is now 

 finished, and hurried and utterly inadequate as it 

 necessarily is, it may have served to indicate that 

 in the present state of knowledge we are able to 

 separate the various components of the animal 

 kingdom into a certain number of divergent groups 

 or phyla ; and that within the limits of these phyla, 



