28 PRIMITIVE ANIMALS 



nuclear material is not sharply divided off from the 

 cytoplasm by a definite membrane, but granules of 

 nuclear material are nevertheless always present and 

 can be readily recognized by their characteristic 

 staining properties. 



The Protozoa show a considerable range of organi- 

 zation, from an almost structureless form like that 

 of Amoeba to the comparative complication of some 

 of the Ciliate Infusoria (Fig. 5 c), which may possess a 

 definite mouth, cuticle, excretory vacuole and stinging 

 threads all within the limits of a single cell. Other 

 Protozoa may attain a very large size and in these 

 cases a great multiplication of nuclei may take place, 

 so that the organism may be multinuclear though 

 not truly multicellular. Again, in many Protozoa, 

 in addition to the simple mode of reproduction by 

 division, a complicated sexual phase may occur in 

 which the individuals are differentiated into male 

 and female gametes in every way comparable to the 

 spermatozoa and ova of the higher Metazoa. There- 

 fore although it may be true that the Protozoa are 

 the simplest known organisms, yet they exhibit all 

 the characteristic phenomena of the higher animals 

 and do absolutely nothing to bridge the gulf between 

 the living and the non-living. Nor in the all-im- 

 portant question of their mode of nutrition do they 

 differ essentially from the higher animals. The Pro- 

 tozoa, including the simplest of them such as Amoeba, 



