CHAPTER VII 

 THE DIGESTIVE AND RESPIRATORY SYSTEMS 



" Wie in jeder Wissenschaft aus den Thatsachen 

 Schliiss sich ergeben, welche das werthvollste 

 Ergebnis der Forschung darstellen, so sind auch fur 

 die vergleichende Anatomic die geistige Verwerthung 

 der Thatsachen durch ihre Verkniipfung das wissen- 

 schaftliche Ziel. Was kann es nutzen, unendlich die 

 Organisation betreffende Erfahrungen zu sammeln, 

 wenn daraus nicht eine Einsicht in jene erwachst, ihr 

 allmahliches Werden verstandlich wird, indem es sich 

 in mannigfachen, aber auseinander hervorgegangenen 

 Zustanden darstellt, die ihre Verwandtschaft unter 

 einander in der Organisation zum Ausdruck kom~ 

 men lassen." 



CARL GEGENBAUR, Lehrbuch d. vergl. Anat. 



1898 ed., p. 27. 



THE first step in the evolution of the Metazoa from pro- 

 tozoan cell colonies, that is, the procedure which initiated the 

 transformation of a colony of similar, one-celled organisms 

 into a single organism of many cells, was the inpushing of its 

 walls at a given point, resulting in the formation of a two- 

 layered cup, the gastrula. From that moment on, the inner 

 and outer cells become placed in a .different position relative 

 to the entire organism, and were thus subjected to different 

 experiences. The outer layer was interposed between the 

 organism and the external world; the inner dealt entirely 

 with the material received into the cavity formed by it and 

 used for food. It is obvious that this difference of experience 

 would result in a physical differentiation of the cells, and such 

 was, indeed, the case. The cells of the outer layer, the ecto- 

 derm, became in part protective and in part receptive of ex- 

 ternal stimuli, differentiations later to result in the formation 

 of an exo-skeleton and a nervous system ; those of the inner 

 layer, the endoderm, developed the power of obtaining, ab- 



