— 14 — 



From this analysis it would appear that the acœlomate 

 animals have little in common with the sponges, cœlenterates 

 and ctenophores, but represent various manifestations of an 

 independent developmental line connecting the infusorians 

 directly with the higher animals in which a number of 

 features inherent or potential in all multicellular organisms, 

 but as we know them mostly confined to plants, have appeared. 



The formation of a multicellular bod}^, giving rise by 

 logical radial development to the sponges, the cœlenterates 

 and the ctenophores, overcame the limitation in size insepa- 

 rable from the unicellular body. The creation of an elongate 

 body form by the development of a multicellular from an 

 infusorian-like type by a gradual multicellular development 

 along animal lines, profoundly modified by the superposition 

 of the characteristic vegetable body form accompanied by the 

 appearance of other plant-like features, led to the differentiation 

 of a head end with a segregation therein of nervous tissues, 

 the formation of sense organs to classify stimuli, voluntary 

 locomotion, and the appearance of an excretory system. 



The acœlomate animals are far more efficient as animals 

 than the cœlenterates, but their superior efficiency is due not 

 to the perfection of the animal type as seen in that group, 

 but on the contrary to the relative imperfection of their 

 organization as animals. 



From the acœlomate animals as a whole, but not from 

 any one of the included types as we know them, the higher 

 animals arose by a crystallization of the developmental processes 

 whereby the excretory and reproductive organs and perivis- 

 ceral cœlome, instead of gradually appearing as independent 

 and disconnected, sometimes even casual, formations in the 

 parenchyma of the more or less grown animal, arise in the 

 early larva typically as evaginations from the enterocœle, 

 correlated with the evolution of a vascular system with a 

 contractile heart. 



With a definite circulatory system the localization of the 

 respiratory function becomes possible, and in conjunction 

 with it we therefore' find a more or less developed respiratory 

 system with special respiratory organs. Furthermore, since 

 all parts of the animal can be reached by the circulatory 

 system and the various organs are no longer dependent for 



