CHAPTER XXI 



i 



PHYLUM CHORDATA 



The phylum Chordata includes all the animals commonly 

 known as vertebrates and a number of others that are 

 somewhat less familiar. All chordates have a thick rod 

 running through the body which is known variously as the 

 chorda dorsalis, chorda, or notochord. This is much like 

 a straight Bologna sausage in form and consistency, having 

 a membrane over the outside and a filling of pulp within. 



integument- , 



muscles of 

 "body H/ar//^**, 



chorda- ,,. 



nervous ^ 

 system 

 -enferon-^ 

 . blood . 



-kidney 



orary 

 coelom 



Fio. 89. Cross sections of an earthworm and a lamprey-eel compared. Note 

 the difference in the relative position of the central nervous system. 



The chorda is strictly a skeletal structure, and has nothing 

 to do with the spinal cord, which is a part of the nervous 

 system. 



Besides the chorda, the chordates have two other dis- 

 tinctive features: (1) the gill slits and the gill arches; and 

 (2) a tubular nervous system situated dorsal to the ali- 

 mentary canal (Fig. 89) . The gill slits are paired openings 

 from the pharynx to the outside of the body. In most 

 aquatic chordates they serve as exits for the water drawn 



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