ZOOLOGY 



SECTION XIII 



PHYLUM CHORDATA 



IN the arrangement which it has been found convenient to 

 follow in the present work, the Vertebrate animals (Fishes, 

 Amphibians, Reptiles, Birds, and Mammals), together with the 

 Cephalochorda or Lancelets, the Urochorda or Ascidians, and 

 the Hemichorda or Balanoglossus and its allies, are all grouped 

 together in a single phylum the Chordata. The main groups 

 comprised in this assemblage, however, differ so widely from 

 one another in certain essential points, and the common features 

 uniting them together are so few, that it has been thought 

 advisable to depart from the plan of arrangement followed in 

 connection with the rest of the phyla, and to make a primary 

 division in this case not into classes, but into sub-phyla. In 

 accordance with this scheme the phylum Chordata is regarded 

 as made up of three sub-phyla the Hemichorda, the Urochorda, 

 and the Euchorda, the last-mentioned comprising the two sections 

 Acrania and Craniata or Vertebrata, each of which receives separate 

 treatment. 



The name Chordata is derived from one of the few but striking 

 common features by which the members of this extensive phylum 

 are united together the possession, either in the young condition 

 or throughout life, of a structure termed the chorda dorsalis or 

 notochord. This is a cord of specially modified vacuolated cells 

 extending along the middle line on the dorsal side of the enteric 

 cavity and on the ventral side of the central nervous system. 

 In the lower Chordates (the Hemichorda, Urochorda, and 

 Cephalochorda) the notochord is developed directly and unmis- 

 takably from the endoderm, and in the first-named group it 

 remains permanently in continuity with that layer. But in the 

 Craniata its origin is by no means so definite, and it may originate 



VOL. II B 



