SECTION XIL PHYLUM CHORDATA 



THE Phylum Chordata comprises all the Vertebrate 

 animals (Fishes, Amphibians, Reptiles, Birds, and Mam- 

 mals) together with the Urochorda or Ascidians : an 

 aberrant group, the Adelochorda (Balanoglossus and its 

 allies) is by many zoologists also included. The name 

 Chordata is derived from one of the most important of the 

 few but striking common features by which the members 

 of this extensive phylum are united together the posses- 

 sion, either in the young condition, or throughout life, of 

 a structure termed the chorda dorsalis or notochord. This 

 is a cord of cells, typically developed from the endoderm, 

 extending along the middle line on the dorsal side of the 

 enteric cavity, and on the ventral side of the central part of 

 the nervous system. It becomes enclosed in a firm sheath 

 and forms an elastic supporting structure. In the Vertebrata 

 (with the exception of Amphioxus, the Lampreys and Hag- 

 fishes, and certain others) it becomes in the adult replaced 

 more or less completely by a segmented bony or cartilagin- 

 ous axis the spinal or vertebral column. Another nearly 

 universal common feature of the Chordata is the perforation 

 of the wall of the pharynx, either in the embryonic or larval 

 condition only, or throughout life, by a system of clefts 

 the branchial clefts ; and a third is the almost universal 



