CHAPTER XVI 

 THE VERTEBRATES 



343. The Backboned Animals. Animals which possess 

 a dorsal vertebral column, which is commonly called " back- 

 bone," are members of the great group Vertebrata. They are 

 popularly called vertebrates or " backboned " animals. It 

 should be noted that the vertebral column in some of the 

 lower fishes is cartilage, not bone; and hence the popular 

 term " backbone " for vertebral column is not strictly 

 accurate. 



The most convenient classification of vertebrates is into 

 five classes : Pisces (fishes) ; Amphibia (frogs and sala- 

 manders) ; Reptilia (reptiles) ; Aves (birds) ; and Mammalia 

 (mammals). 



It has been'discovered within the past half century that some 

 existing animals lower than the fishes are certainly related 

 to the vertebrates, even though they do not have a back- 

 bone. However, these low forms do have a stiffening rod 

 in place of the backbone, and it is similar to a rod which 

 is present in the embryos of all backboned animals as the 

 axis around which the backbone develops. This rod is 

 called the chorda dorsalis (meaning dorsal cord) or notochord, 

 and all animals which possess it in any stage of their existence 

 are called chordates. The recent zoological books recognize 

 a Division Chordata, including (1) the very simple forms with 

 the chorda, but having no vertebral column and skull, and 

 (2) the vertebrates. In this course we cannot study in 

 detail any of the simple chordates ; but museum specimens 

 of ascidians (sea-squirts), Amphioxus (lancelet), and the 

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