PRIMITIVE MULTICELLULAR ANIMALS 



However, their peculiarities of organization and their simpHcity clearly in- 

 dicate that, phylogenetically, they antedate all other members of this sub- 

 kingdom. (In this statement we are assuming that the Mesozoa are primarily 

 simple, and not secondarily simplified organisms.) Both may be assumed to 

 have arisen from somewhere near the point of origin of multicellular animals 

 from unicellular ancestors, and the history of both groups, particularly of the 

 sponges, is marked by a certain amount of evolutionary progress. This 

 progress has advanced in different directions in the two groups, but in neither 

 has the path led in the direction taken by more progressive animals. No 

 metazoans other than Mesozoa, for example, possess as their only internal 

 tissue a cell mass specialized exclusively for reproduction; none but the 

 sponges has as its only internal cavity a system of water chambers lined by 

 choanocytes. 



It therefore seems reasonable to conclude that the three simplest metazoan 

 phyla, Mesozoa, Porifera, and Coelenterata (which will be discussed in the 

 next chapter), represent groups which evolved independently, and at different 

 times, from primitive multicellular ancestors. Of the three fundamental body 

 plans characteristic of these groups, those of the Mesozoa and Porifera were 

 evidently so limited in evolutionary possibilities that only the modern repre- 

 sentatives of these same phyla can be traced to them. The basic coelenterate 

 plan, with an internal digestive tissue later excavated to form a hollow 

 enteron, was apparently sufficiently adaptable to the needs of animals to serve 

 as the basis for further evolution. 



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