SECTION XII.— PHYLUM CHORDATA 



The Phylum Chordata comprises all the vertebrate ani- 

 mals ('fishes, amphibians, reptiles, birds, and mammals), 

 together with the Urochorda or Ascidians, and the Adelo- 

 chorda 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 only of cells, typically developed from the entoderm, 

 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 and the lam- 

 preys and hag frsrres) it becomes in the adult replaced more 

 or less completely by a segmented bony or cartilaginous 

 axis — the spinal or vertebral column. Another nearly uni- 

 versal 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 ncurocak, in- llic iuluiui uf thu budy, 

 -ly ing abuvc the central nervous system. 



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