CHAPTER XXIII 



PHYLUM CHORDATA 



Phylum Chordata (kor da' ta, cord) is made up of the group of 

 animals which includes man himself and in general the more con- 

 spicuous, better known animals. 



Characteristics 



There is a rather wide range of variation as to form and size in 

 the group. It includes minute sessile forms, small colonial forms, 

 mud-burrowing forms, and on up to the largest and most complex 

 of living animals. All individuals classified in the phylum possess 

 three distinctive characteristics that are most conspicuous in cer- 

 tain primitive forms. The three features clearly distinguish the 

 phylum from all others and bind together individuals which are 

 widely separated in appearance but characterized by certain traits 

 peculiar to this group alone. These three characteristics are: (1) 

 noiochord, a flexible rod extending from anterior to posterior in the 

 longitudinal axis of the body, lying dorsal to the digestive tube and 

 ventral to the nerve cord; (2) pharyngeal clefts or gills, a series of 

 paired slits in the wall of the pharynx and in the body wall of some ; 

 (3) dorsally located tiibular nerve cord, extending the length of the 

 body dorsal to the notochord and other organs. 



The notochord serves as a stiffening rod and is the foundation axis 

 for the endoskeleton. It is present as such at some time during the 

 life of every cliordate animal. In the adult vertebrate it is replaced 

 by the centra of the vertebrae. The gill clefts are present at some 

 time in the life of all individuals placed in this phylum. Although the 

 gills become modified to form other structures in the adult terrestrial 

 chordates including man, they have had rather typical ones as em- 

 bryos. The pharyngeal clefts or gills provide a more effective mode 

 of respiration for aquatic animals than that used by most non- 

 chordates because the gills are thus interposed directly in the 

 course of the circulation, and the entire blood supply of the body 

 passes through them. The central nervous system is derived from 

 the ectoderm along the middorsal line of the embryo, first as a 

 plate, then as a groove, and finally a tube which results in the spinal 



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