CHAPTER II 

 METAZOA INTRODUCTORY REMARKS 



the phylum Protozoa we had to do with single cells living inde- 

 pendently. The individual cell multiplies from time to time, by fission 

 or otherwise, but the cells so arising separate and lead an independent 

 ,-nce like the parent. In all the members of the animal kingdom 

 1'rotozoa commonly grouped together under the name 

 XZOA the life-history commences with a stage in which the indivi- 

 le cell a zygote which as in the case of the Protozoa 

 mul; a process of fission repeated over and over again, but in 



case the successive generations of cells produced by the process of 

 fission do not break apart and lead an independent existence. On the 

 remain as a coherent mass of cells which, in correlation with 

 fission of its component cells, exhibits growth in size. Just 

 i the case of the Protozoa, the process of fission slackens off in due 

 that the cell-mass does not increase in size indefinitely but 

 K to a more or less definite full-grown size. This mass of 

 cells forms the body of the Metazoon, an individual of a higher order 

 ndividual seen in the Protozoa, for it is composed of a mass 

 of cells which cohere together and have their individualities merged in 



! tin- whole. 



In mple Protozoon the cell-individuals are unspecialized ; 



earl like its forebears. In the body of the Metazoon on the 



il the successive generations of cells which come into existence 



dun iiMing up of the fully developed body become more and 



m( >rc S|M : they gradually lose the primitive unspecialized 



character of their zygote ancestor and, with the loss of the unspecialized 



r. they lose for the most part their capacity for con- 



J U K process of syngamy with other cell-individuals. At one 



the Ix.dy however there remain nests of cells which do 



not become side-tracked on any path of specialization for particular 



8 4 





