12 



COMPARATIVE ANATOMY 



CHAP. 



The Protozoa are unicellular organisms, or simple colonies of 

 similar unicellular organisms. The typical character of a unicellular 

 organism often appears disturbed by the presence of more than one 

 nucleus, the original simple nucleus by successive division separating 

 into several or even many nuclei. These divisions of the nucleus are 

 in some cases connected with reproduction, as its first stage, in others 

 the rest of the cell remains altogether unaffected. 



Although the Protista are unicellular organisms they show a 

 remarkable variety in form, and in some cases a great complexity of 



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Fig. 19.— Amcelja polypodia. In the successive stages of division. The light spot is the 

 contractile vacuole, the dark spot the nucleus (after F. E. Schulze). 



structure. Modifications may arise specially adapted to the most 

 varied functions of life ; these, however, unlike the modifications in 

 the Metazoa, are always in one and the same cell. Nowhere in the 

 organic world does the cell reach so high a degree of morphological 

 diiferentiation as in certain Protozoa. In the lower Protozoa, on the 

 other hand, the cell in its simplest form shows itself capable of all 

 essential life processes. An Amixha (Fig. 19) may consist of a small 

 mass of uniformly granulated protoplasm containing a nucleus. 

 Locomotion is caused by the streaming forward of the protoplasm at 

 some points and the consequent formation of processes of varying 



