458 COMPARATIVE ZOOLOGY 



the lowest of the Metazoa, long considered to be col- 

 onies of Protozoa, so ill defined are their layers of tissue; 

 the two-layered coelenterates whose bodies contain but 

 a single cavity with one opening serving for ingestion 

 and egestion; the "worms," with a body consisting, 

 except in some of the parasitic and degenerate forms, 

 of three layers of tissue (ectoderm, mesoderm, and 

 endoderm), with an alimentary canal having both inlet 

 and outlet, a well-defined nervous system, and, in the 

 higher orders, a segmented body ; the arthropods, with 

 their segments showing a tendency to become grouped 

 into distinct regions, with jointed appendages for per- 

 forming functions, and with respiratory organs, in the 

 higher groups, for breathing air directly ; the verte- 

 brates, beginning with forms which have the merest 

 trace of a notochord, and progressing through the lower 

 fishes with cartilaginous skeletons, small brains, and 

 two-chambered hearts, to the amphibia and reptiles, with 

 bony skeletons, larger brains, and three-chambered 

 hearts, and finally to the warm-blooded birds and mam- 

 mals, with four-chambered hearts, and with brains and 

 nervous systems far superior in size, structure, and 

 function to those of all other groups. This hasty 

 review does not, by any means, take cognizance of all 

 the structural features that might be mentioned, but 

 only draws attention to some of the most obvious char- 

 acters which show progressive change from lower to 

 higher forms. 



The metameric arrangement of the bodies of the 

 annulata, in which each segment is a more or less per- 

 fect repetition of the preceding and succeeding seg- 

 ments, is again recognizable, though less plain, in certain 

 structures in the bodies of crustaceans and insects, and 

 is very obvious in the chordates from fishes to man, as 

 the vertebrae and pairs of ribs, the muscle plates and 



