CHAPTEE XXI 

 CHORDATA 



C Enteropneiista — Balanoglossvs. 

 j Hemichordata-! Cephalodiscida — Cephalodiscus. 

 \ Rhabdopleurida — Hhdbdopleura. 



I Larvacea — Appendieularia. 



CHORDATA^UKOCHOKBATA(Tunicata) Sa^r-cfoiri^^^^^ 



' soma. 



I Cephalochordata — Amphioxus. 



V:ektebrata. 



I. Hemiohoedata (Enteropneusta, etc.) 

 II. Urochoedata (Ascidians or Tunicata), 



III. Cephalochoedata. 



IV. Veetebeata. 



Chaeacteristics. — The phylum Chordata consists of coelomate 

 animals characterised iy (i.) the possession of gill-slits or 

 apertures leading from the pharynx to the exterior, these 

 may persist through life, or they may he hit temporary 

 openings existing only in the embryo; (ii.) ly the presence 

 of a skeletal rod, the notochord, which is formed in the dorsal 

 middle line by cells budded off from the hypoblast of the 

 embryo ; (iii.) ly a dorsal nervous system, which, with a few 

 exceptions, is tubular and does not form a nervous ring 

 round the alimeniary canal; (iv.) by the segmentation of 

 the mesoblast, which, although often obscured in later life, 

 is always found in the earliest stages. 

 From some points of view the Chordata is the most im- 

 portant of the various phyla which make up the animal 

 kingdom. The first three groups are either degenerate or 



