CHAPTER XVI 

 THE VERTEBRATES 



The backboned animals or vertebrates, coniprising the 

 fishes, salamanders, frogs and toads, lizards, crocodiles, 

 turtles and snakes, birds, and all the quadrupeds or mam- 

 mals, belong to the great branch Chordata which includes 

 also a few small unfamiliar ocean animals which do not 

 look at all like the backboned animals, but which agree 

 with them in possessing a peculiar structure called the 

 notochord. This notochord consists of a series or cord of 

 cells extending longitudinally through the body from head 

 to tail, above the alimentary canal and below the spinal 

 nerve-cord. In all the vertebrates excepting a few low 

 forms, the notochord although present in the young, is replac- 

 ed in the adult by a segmented bony or cartilaginous axis, the 

 spinal or vertebral column. But in the ascidians or sea- 

 squirts (called also tunicates) it persists throughout life. 

 In addition to this characteristic notochord, nearly all the 

 Chordata are marked by the presence, either in embryonic 

 or larval stages only, or else persisting throughout life, of a 

 number of slits or clefts in the walls of the pharynx which 

 serve for breathing, and which are called gill-slits. 



Structure of the vertebrates. — As the backboned or 

 vertebrate animals make up almost the whole of the branch 

 Chordata, and as the few other chordates are animals the 

 special structures of which we shall not undertake to study 

 in this book, we may note here some of the other more 

 obvious structural characteristics of the true vertebrates. 

 The possession of a backbone or bony (sometimes cartilagi- 



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