METAZOA, METAPHYTA AND PROTISTA 4 1 



are sharply marked off; the Metaphyta, or Higher Plants, which 

 it is not so easy to characterise, but which unite at least two or 

 more vegetal characters ; and the Protista, or organisms, whose 

 differentiation is limited to that within the cell (or apocyte), and 

 does not involve the cells as units of tissues. These Protista, 

 again, it is impossible to separate into animal and vegetal so 

 sharply as to treat adequately of either group without including 

 some of the other : thus it is that every text-book on Zoology, 

 like the present work, treats of certain Protophyta. The most 

 unmistakably animal group of the Protista, the Ciliata, is, as we 

 have seen, by the complex differentiation of its protoplasm, 

 widely removed from the Metazoa with their relatively simple 

 cells but differentiated cell-groups and tissues. The line of 

 probable origin of the Metazoa is to be sought, for Sponges at 

 least, among the Choanoflagellates (pp. 121 f. 181 f.). 



