CHAPTER XXI 



THE ANCESTRY OF VERTEBRATES 



Somewhere out of the great variety of phyla of invertebrates 

 the vertebrate stem arose. The origin of this brancli of the 

 animal kingdom has a special interest for us because to it 

 belong not only man but also the most of our favorite fellow- 

 creatures, our domestic animals, the birds, and the fishes. We 

 have now to consider what is known of the ancestry of verte- 

 brates. 



In seeking the origin of any phylum we must first sejiarate 

 out what is essential and universal in it from what is secondary 

 and special. And first of all, the phylum of vertebrates is 

 distinguished by the possession of an internal skeleton whose 

 simplest forerunner is a rod of tissue lying in the axis of the 

 body and running from the head to the end of the tail. This 

 rod is called the chorda. It is found in some simple animals 

 which have no other internal skeleton, and so they and verte- 

 brates are often classed together as Chordates. The essential 

 feature of CUiordates are four: (I) the chorda just referred to; 

 (2) gill-slits passing through the wall of the throat so that 

 water taken into the mouth passes out through the neck, as is 

 seen in fishes ; (3) a nervous cord that lies wholly on the dorsal 

 side of the animal ; and (4) a heart which lies on the ventral 

 side. Any animal which can show this combination of charac- 

 ters, or any of them, thereby reveals its affinity to the verte- 

 brates, no matter how lowly it may be. 

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