ZOOLOGY 



SECTION XIII 

 PHYLUM CHORDATA. 



THE Phylum Chordata comprises all the Vertebrate animals 

 (Fishes, Amphibians, Reptiles, Birds, and Mammals) together with 

 the Urochorda or Ascidians and the Adelochorda or Balanoglossus 

 and its allies. 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 

 possession 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 nervous system. It becomes 

 enclosed in a firm sheath, and forms an elastic supporting 

 structure. ) In the Vertebrata (with the exception of Amphioxus 

 and the Lampreys and Hag-fishes) it becomes in the adult replaced 

 more or less completely by a segmented bony or cartilaginous 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 presence at all stages, or only in 

 the larva, of a cavity or system of cavities, the neuroccele, in the 

 interior of the central nervous system. 



SUB-PHYLUM AND CLASS I, ADELOCHORDA. 



Until quite recently a single genus, Balanoglossus, was the only 

 known representative of a class to which the name JEnteropneusta 

 was applied. There seems reason to believe, however, that two 

 remarkable deep-sea animals 1 Rhabdopleura and Cephalodiscus 

 though not close allies of Balanoglossus, may yet be sufficiently 

 nearly related to^i-t to justify their being placed in the same class. 



VOL. II B 



