76 DIVERGENCE AND PARALLELISM 



constituting the master tissue of the animal 

 body, and there is no doubt that he is right 

 up to a certain point. The behaviour of 

 Protozoa and Sponges in this regard, however, 

 teaches us that in the course of evolution the 

 nervous mechanism is secondary as compared 

 with the mechanism of locomotion on the one 

 hand, and nutrition on the other. An obvious 

 rejoinder to this argumentation might be that 

 the organisation of Protozoa and Sponges also 

 teaches us that the gut, as an organ, is secondary 

 to nutrition as a function. That this is so is 

 borne out by the fact, referred to above, that 

 just as in plants certain structures which are not 

 leaves have secondarily acquired the function 

 of leaves, so amongst animals the parasitic 

 Cestode and Acanthocephalous worms, which 

 have secondarily lost the use and presence of 

 a gut as a consequence of their mode of life, 

 absorb fluid nutriment, prepared by the host, 

 through the outer surface of the body. Here 

 therefore the body-wall does duty for the gut- 

 wall, combining the somatic and splanchnic 

 functions into one. 



We are therefore driven to enquire whether 

 the general homology of the gut of triploblastic 

 (three-germ-layered) animals still retains its 

 dominance, like that of the foliage leaf, in spite 

 of exceptional cases and special homologies. 

 The only reply to this imaginary question that 



