402 CHORD AT A. 



CHAPTER XXIV. 

 GENERAL FEATURES OF CHORD ATA. 



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 Tunicata to Man. It has five 

 leading structural characteristics which are present through- 

 out the group at one time in the life of each individual. 



{}) A hollow dorsal nerve-tube^ the anterior end of which 

 is hypertrophied to form the brain. It arises from the 

 epiblast. 



(2) The 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, i,e,, forwards ventrally and backwards 

 dorsally. 



All the other phyla differ from Chordata in these 

 characters and they are often contrasted with them as 

 Non- Chordata. 



It will be remembered that certain of the Ccelenterata present gastro- 

 vascular pouches which appear to be incipient coelomic pouches. In 

 the functions performed by their walls and in their hypoplastic 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. In a similar way certain of the Non- Chordata^ namely, 

 a class of the Arckiccelof?iata, called Archichorda (or Hemichordd), show 

 several of the chordate characters in an incipient stage. The type of 

 Archichorda described [i.e.^ Balanoglossus) shows a series of pharyngeal 

 clefts not essentially differing from those of Amphioxus, and these are 



