CHAPTEE XIII 



THE CHORDATES 



149. General characters. Up to the present time we have 

 been studying the representatives of a vast assemblage of 

 animals whose skeletons, if they have any at all, are located 

 on the outside of the body. In the corals, the mighty com- 

 pany of arthropods, and the echinoderms, it is external. On 

 the other hand, we shall find that the animals we are now 

 about to consider, the fishes, frogs, lizards, birds, and mam- 

 mals, are in possession of an internal skeleton. In some of 

 the simpler fishes and in a number of more lowly forms (Fig. 

 99) it is exceedingly simple, and consists merely of a gristle- 

 like rod, the notochord (Fig. 101, nc), extending the length 

 of the body and serving to support the nervous system, which 

 is always dorsal. This is also the type of skeleton found in 

 the young of the remaining higher animals, but as they grow 

 older the notochord gives way to a more highly developed 

 cartilaginous or bony, jointed skeleton, the vertebral column. 



In the young of all these back-boned or chordate ani- 

 mals, the sides of the throat are invariably perforated to 

 form a number of gill-slits. In the lower forms these per- 

 sist and serve as respiratory organs, but in the higher ani- 

 mals they disappear in the adult. The chordates are thus 

 seen to be distinguished by the possession of a dorsal nerv- 

 ous cord supported by an internal skeleton and by the 

 presence of gill-slits, characters which separate them widely 

 from all invertebrates. 



The chordates may be divided into ten classes, seven of 



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