CHAPTER VII 



CHOBDATES IN GENERAL 



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. 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. 



Characteristics 



All individuals classified in the phylum possess three distinctive 

 characteristics that are most conspicuous in certain primitive forms. 

 The three features clearly distinguish the phylum from all others 

 and bind together individuals which are widely separated in appear- 

 ance but characterized by certain traits peculiar to this group alone. 

 These three characteristics are: (1) notochord, a flexible rod ex- 

 tending 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 tuhulai^ 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 chordate 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 

 cord and brain. In higher forms the anterior end of the tube be- 



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