Chap. XIX.] VORTICELLA AND PARAMCECIUM. 353 



but one (c. v.). The vestibule (vs.) in nyctotherus is very con- 

 spicuous, and on its lower edge there is a long stiff seta; the 

 anus also forms a distinct cleft (an.). 



The fact to be especially noted in the forms which we have 

 studied in this Chapter is that they are unicellular organisms, or 

 protozoa, are never possessed of sexual organs, and are thus 

 very markedly distinguished from the multicellular organisms 

 or metazoa. Metazoa are typically produced by a segmentation 

 in the ovum, by which a number of cells result, which remain 

 so connected as to be constituents of a single individual. 

 Differentiation in the metazoa proceeds by the divergent de- 

 velopment of different cells, a morphological differentiation 

 of structure accompanying a physiological differentiation of 

 function. In the higher forms the differentiated cells aggregate 

 into groups, and thus form tissues, which take their part 

 in the formation of more or less differentiated organs. In the 

 protozoa there is no segmentation of the ovum. The process of 

 fission does, indeed, in some respects resemble the first cleavage 

 of the ovum into two blastomeres, but it also serves to show the 

 fundamental distinction between the two groups of organisms. 

 For in the protozoa, after fission, each cell typically becomes a 

 distinct individual. There is thus no opportunity for that 

 physiological division of labour which is normal among the con- 

 stituent cells of the metazoan. 



Differentiation, therefore, where it goes on among the protozoa, 

 must take place within the single cell of which each individual 

 is framed. The forms we have studied show the nature of this 

 differentiation. At the best they remain lowly forms of life. 

 They have not learnt the secret of merging their individuality 

 for the common good, and being content to be the constituent 

 units, each with its own narrow field of work, of a nobler 

 organism. 



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