CHAPTER XIV 



Cyclostomata 



Craniata. — The Craniata include the vertebrates, from the eel- 

 like lamprey, up to man himself. In all we find that in the embryo 

 an axial notochord appears. This is persistent inside the centrum 

 of the vertebra of an elasmobranch, but is replaced in higher forms 

 by the true vertebral column. 



Classification. — 



Characteristics of Craniata. — i. Segmented animals without 

 external ringing of the body but with metameric arrangement of the 

 internal parts. 



2. A cuticular skeleton absent, but there may be a horny epi- 

 thelium or dermal ossifications. (Scales, etc.) 



3. An axial skeleton is present; either a notochord or skull and 

 vertebral column. Two kinds of appendages are supported by the 

 axial skeleton, the unpaired fins of the fishes and amphibia and the 

 paired fins, or limbs of the higher vertebrates. 



4. The central nervous system is dorsal and hollow and consists 

 of cerebrum, midbrain, optic lobes, cerebellum, and medulla ob- 

 longata, with the spinal cord attached. The eyes and ears are the 

 most highly developed of the sense organs. 



5. The respiratory organs arise from the endoderm; gill slits are 

 present in the embryo. In land forms, these are replaced by lungs, 

 developed from the hinder part of the pharynx. 



6. The heart is ventral and consists of one or two auricles; and 

 one or two ventricles. In gill breathers the blood in the heart is 

 venous. Pulmonary respiration brings blood to the heart pure 



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