CHAPTER XXI 
CHORDATA 
Enteropneusta—Balanoglossus. 
HeMICHORDATA 4 Cephalodiscida—Cephalodiscus. 
Rhabdopleurida—Rhabdopleura. 
Larvacea—A ppendicularia. 
. Thaliacea—Doliolum, Salpa. 
CHORDATA < Urocuorbara (Tunicata) Wacidinecs =! Olona, ‘Botr#llue, Pyro: 
soma. 
CEPHALOCHORDATA—A mphioxus. 
VERTEBRATA. 
I. Hemicuorpata (Enteropneusta, etc.) 
II. Urocuorpata (Ascidians or Tunicata). 
III. CEPHALOCHORDATA. 
IV. VERTEBRATA. 
CHARACTERISTICS.— The phylum Chordata consists of coclomate 
animals characterised by (i.) the possession of gill-slits or 
apertures leading from the pharynx to the exterior, these 
may persist through life, or they may be but temporary 
openings existing only in the embryo; (ii.) by 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.) by a dorsal nervous system, which, with a few 
exceptions, is tubular and does not form a nervous ring 
round the alimentary 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 
