CHAPTER II 



MORPHOLOGY AND PHYSIOLOGY OF PROTOZOA 



PROTOZOA range in size from less than 5 microns to more 

 than 1 cm. in diameter. On the whole, however, they are 

 microscopic. The parasitic forms, especially the cytozoic 

 parasites, are generally small, while the free-living forms are 

 of much larger dimensions. Leishmania (Fig. 52) and Plas- 

 modium (Fig. 121) may be taken as examples of minute Proto- 

 zoa. There are numerous species of free-living ciliates which 

 can be seen with the unaided eye, and these are among the larg- 

 est Protozoa. 



The fundamental components of the protozoan body are the 

 nucleus and the cytoplasm. Not a single protozoan is known 

 which does not possess at least one nucleus. Experimental evi- 

 dence indicates clearly that when a protozoan is deprived of its 

 nucleus it degenerates sooner or later. On the other hand, if the 

 loss is in the cytoplasmic portion, the organism is capable of 

 regenerating its lost part. Thus the nucleus controls both ana- 

 bolic and catabolic activities of the organism. Furthermore, the 

 nucleus contains chromatin substance in which are lodged 

 "genes," or factors, characteristic of each species. 



The Nucleus 



The nucleus is of various form, size, and structure. At one 

 extreme there is a small compact nucleus and at the other a 

 large moniliform or elongated one. In between is found every 

 conceivable variety of form. The majority of Protozoa con- 

 tain a single nucleus. During the process of division, however, 

 there may be two or more nuclei. There are, moreover, forms 

 which regularly contain two or more nuclei throughout the 

 greater part of their life-cycle. In several widely scattered spe- 

 cies, each individual possesses two similar nuclei, for example, 

 in Pelomyxa binucleata, Arcella vulgaris (Fig. 4), Giardia intes- 

 tinalis (Fig. 62), Dientamoeha fragilis (Fig. 87), etc. In several 



[201 



