CHAPTER XIII 



Chordata 



Classification 



Subphylum Hemichorda (Enteropneusta). 

 Subphylum Urochorda (Tunicata or Ascidia). 

 Subphylum Adelochorda (Acrania). 

 Subphylum Vertebrata (Craniata). 



Class Cyclostomata. 



Class Pisces. 



Class Amphibia. 



Class Reptilia. 



Class Aves. 



Class Mammalia. 



The Phylum Chordata Includes not only common Vertebrate 

 animals such as Fishes, Amphibia, Reptilia, Birds and Mammals, 

 but also includes certain marine forms that are extremely significant 

 in discussions of the probable origin of Vertebrates from Inverte- 

 brates. 



The name Chordata is derived from the notoc/iord, which is 

 found at some stage of the existence of all Vertebrates, but is per- 

 sistent in the adults of those intermediate types that we will shortly 

 proceed to describe. The notochord is a long, cylindrical, double- 

 pointed rod, which is located ventral to the nerve cord. Notochor- 

 dal tissue consists of large vacuolated cells (somewhat resembling 

 the pith of plants) with their nuclei confined to the dorsal and 

 ventral regions. The vacuoles of these cells are filled with liquid 

 which renders them turgid. The notochordal sheath is extended 

 dorsally to enclose the nerve cord. 



Comparison of Vertebrates and Invertebrates 



Up to the present time we have been dealing entirely with the 

 Invertebrates. It is therefore desirable, before we go further with 

 the animal phyla, to compare briefly the Invertebrates with the 



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