CHAPTER 



9 



MULTICELLULARITY, 



AND THE SIMPLEST 



METAZOAN ANIMALS: 



Mesozoa and Porifera 



In Chapter 8 we discussed representatives of the phylum Protozoa, modern 

 descendants of the primitive one-celled animals; these have been limited 

 in their specializations to the same one-celled plan of organization which 

 must have characterized their remote ancestors. Earlier, it was pointed out 

 that a significant epoch in the evolutionarv historv of animals was marked by 

 the rise of forms consisting of manv cells, each cell an integral unit playing 

 its part in the economy of the individual, and in its special activities sub- 

 servient to the whole complex. Without question, the evolution of multi- 

 cellularity opened vast new potentialities and made possible the development 

 of the Animal Kingdom as we know it. In view of the fundamental similari- 

 ties demonstrated between the vital functions of Protozoa and those of higher 

 animals, it appears most reasonable to assume that Protozoa and Metazoa had 

 a common ancestry, and that the progenitors of both lines were one-celled 

 organisms. 



In this chapter we shall consider the manner in which many-celled animals 

 may be thought to have arisen from these unicellular forebears, and some 

 implications of the multicellular condition for the further progress of the 

 Metazoa. In addition, we shall discuss briefly two of the modern phyla, the 

 Mesozoa and the Porifera. These groups are .so unlike the true Metazoa that 

 they have been placed in separate branches, the Mesozoa and the Parazoa 



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