CHAPTER 29 



THE PHYLUM CHORDATA 



THE PROTOCHORDATES AND FISHES 



Though certainly not the most abundant of all animal phyla so 

 far as numbers are concerned, the chordates are both the most familiar 

 and economically important. This phylum contains not only man, but 

 all his domesticated animals, the familiar birds, reptiles, and amphibians, 

 and the numerous and varied fish. Also included within this phylum 

 are a few small marine forms which economically are of no importance, 

 but biologically of great interest. 



Although not nearly so abundant as the arthropods, the chordates 

 rival them in their diversity of form and adaptation. Like the arth- 

 ropods, the chordates have invaded all terrestrial habitats, fresh water, 

 the ocean, and, like the insects, the air itself. Except for some small 

 marine forms, the chordates have some semblance of a vertebral column ; 

 hence most are known as the vertebrates. The development of this 

 dorsal supporting structure together with an internal skeleton has per- 

 mitted the vertebrates to attain relatively large sizes. In general, they 

 are much larger than the members of the many invertebrate phyla, with 

 some such as the whale attaining gigantic proportions. 



Embryologically the chordates show their relationship to the echino- 

 derm line of development. They exhibit indeterminate cleavage, meso- 

 derm derived from the roof of the archenteron, and a blastopore which 

 develops into an anus ; however, the echinoderms, the chaetognaths, and 

 even the hemichordates are so different that they cannot be considered to 

 be direct ancestors of the chordates. Rather, they, too, are highly spe- 

 cialized forms diverging from this branch of development. 



The problem of the origin of the vertebrates from some invertebrate 

 ancestor has occupied the attention of many zoologists. Numerous 

 theories have been developed, deriving them from such divergent forms 

 as the annelids, arthropods, arachnids, echinoderms, and virtually every 



559 



