CHAPTER 21 



SOME OF THE LOWER METAZOANS 



While the members of the subkingdom Protozoa are very diverse 

 and numerous, those members of the subkingdom Metazoa are even 

 more diverse and certainly as numerous. This subkingdom includes 

 not only those forms barely more complex than the protozoans in their 

 organization, but also the highly specialized vertebrates and insects. 



In the classification of the metazoans, three major branches are 

 recognized : the Mesosoa, the Parazoa, and the Eumetazoa. Both the 

 Parazoa and the Mesozoa represent a very low grade of organization. 

 More highly developed than either of these two branches are the 

 many phyla which show tissue differentiation. These constitute the 

 Eumetazoa whose members represent the bulk of the animal kingdom. 

 Two of the latter phyla, the Coelenterata and the Ctenophora, shall be 

 considered in this chapter along with the Mesozoa and the Parazoa. 



THE PHYLUM PORIFERA 



The phylum Porifera, containing the sponges, is the only one in the 

 branch Parazoa. The name Parazoa, meaning animals to the side, 

 aptly describes these unusual animals. In most respects they bear little 

 or no resemblance to the rest of the metazoans. True, they do have 

 some cellular differentiation, but there is no real tissue development. 



H. V, Wilson in 1907 demonstrated the primitive organization of 

 the sponges. He squeezed a sponge through very fine silk cloth, separat- 

 ing it into individual cells and cell masses. The resulting mass of sepa- 

 rated material was able to reassemble and by cellular differentiation 

 form a new sponge like the original. Of course, this type of treatment 

 would destroy any eumetazoan form. The anatomy, reproduction, and 

 embryology further demonstrate the fact that these sponges are totally 

 unlike other metazoans. 



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