402 CHORDATA. 
CHAPTER XXIV. 
GENERAL FEATURES OF CHORDATA. 
PHYLUM CHORDATA. 
The Phylum Chordata is in many respects the most 
important of the whole animal kingdom and contains an 
infinite variety of types from Zunicata to Man. It has five 
leading structural characteristics which are present through- 
out the group at one time in the life of each individual. 
(1) A hollow dorsal nerve-tube, the anterior end of which 
is hypertrophied to form the brain. It arises from the 
epiblast. 
(2) Lhe primary skeletal axis or notochord, an elastic rod 
of chordoid tissue lying under the nervous system and 
arising from the hypoblast. 
(3) Paired pharyngeal clefts formed from protrusions of 
the hypoblast in the anterior region of the alimentary canal. 
(4) A metameric segmentation of the mesoblast, obscure 
only in the lowest class. : 
(5) A ventral heart or contractile circulatory organ (which 
may be multiple, as in Amphioxus), and a particular course 
of the blood-system, z.¢., forwards ventrally and backwards 
dorsally. 
All the other phyla differ from Chordata in these 
characters and they are often contrasted with them as 
NNon- Chordata. 
It will be remembered that certain of the Celenterata present gastro- 
vascular pouches which appear to be incipient coelomic pouches. In 
the functions performed by their walls and in their hypoblastic origin 
they agree with the latter, but they are not completely separated from 
the gastric cavity and hence are not regarded as forming a third layer 
or mesoderm. Ina similar way certain of the Mon-Chordata, namely, 
a class of the 4rchicelomata, called Archichorda (or Hemichorda), show 
several ‘of the chordate characters in an incipient stage. The type of 
Archichorda described (2.e., Balanoglossus) shows a series of pharyngeal 
clefts not essentially differing from those of Amphioxus, and these are 
