CHAPTER X. 

 BRANCH CHORDATA. 



THIS branch is mainly composed of the vertebrates, or 

 backboned animals, that is, fishes, amphibians, reptiles, birds, 

 and mammals. But it is now found necessary to class with 

 them certain other animals formerly regarded as inverte- 

 brates. Hence the old branch Vertebrata is made a sub- 

 branch, and, with two other subbranches, included in the 

 branch Chordata. The chordate animals are characterized 

 by the possession of a dorsal chord or notochord. This is 

 a supporting rod extending along the dorsal region between 

 the body cavity and the main nervous system or spinal 

 cord. While the notochord is always present in the young, 

 it is, with a few exceptions, replaced in the adult by a seg- 

 mented cartilaginous or bony axis, which is known as the 

 spinal or vertebral column. In other words, the notochord 

 is a sort of forerunner of the backbone. 



Subdivisions of Chordata. The branch Chordata is 

 divided into three subbranches : 



1 . Adelochorda, wormlike, marine forms (Balanoglossus). 



2. .Urochorda, the tunicates or ascidians. 



3. Vertebrata, lancelet to mammals. 



Division A. Acrania, the lancelets. 



(a) Cyclostomata, without 

 jaws (lampreys). 



Division B. Craniata. 



(b) Gnathostomata, with 



jaws (true fishes to 

 mammals). 



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