CHAPTER XXIII 



BRANCH CHORDATA: THE VERTEBRATES, 

 ASCIDIANS, ETC. 



THE branch Chordata includes all the backboned 

 animals or vertebrates, comprising the fishes, salamanders, 

 frogs and toads, lizards, crocodiles, turtles and snakes, 

 birds, and all the quadrupeds or mammals, and 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 while present in the young, is re- 

 placed 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 through- 

 out 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 under- 

 take to study in this book, we may note here some of the 

 other more obvious structural characteristics of the true 



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