CHAPTER IX 

 SURVEY OF VERTEBRATES 



The wise man wonders at the usual. — Emerson. 



Now that we have viewed the important phyla of the lower 

 animals, it remains to survey the highest and concluding phylum 

 of the Animal World, technically known as the Chordata, which 

 for all practical purposes is synonymous with the Vertebrata. 

 The only Chordates that are not Vertebrates, or backboned 

 animals, are a few lowly creatures apparently having Invertebrate 

 and certainly Vertebrate affinities; the latter chiefly evidenced by 

 the presence of a notochord which is the forerunner of the back- 

 bone, a dorsal nerve tube, and gill slits for respiration. Among 

 these primitive Chordates the most interesting is Amphioxus be- 

 cause it is most closely related to the Vertebrates. (Fig. 67.) 



It will suffice for us, then, to proceed directly to the true Ver- 

 tebrates, and since emphasis is later to be placed on the anatomy 

 and physiology of the Vertebrate body because Man is a Verte- 

 brate, our immediate attention can be largely confined to their 

 classification. 



The Vertebrates include all the larger and more familiar animals 

 — Fishes, Amphibians, Reptiles, Birds, and Mammals — so that 

 in the popular mind the words animal and Vertebrate are essen- 

 tially synonymous. A Fish, as everyone knows, is an aquatic back- 

 boned animal which breathes by means of gills and moves by fins. 

 An Amphibian, such as a Frog, may, in a general way, be thought 

 of as a Fish which early in life — at the end of the tadpole stage — 

 discards its gills, develops lungs, substitutes five-toed limbs for 

 fins, and takes up a terrestrial existence. Similarly, a Reptile, say 

 a Lizard, may be pictured as an Amphibian which has relegated, 

 as it were, the tadpole stage to the egg, and therefore emerges 

 with limbs and lungs. Birds and Mammals may be regarded as 

 separate derivatives of the reptilian stock which have transformed 

 the scales of the Reptile into feathers and hair respectively, and 

 have developed a special care for their young: the Birds by incu- 

 bation of the eggs and the Mammals by retention of the young 



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