4 OEGANIZATION AND LIFE-HISTORY OF PROTOZOA 



which cannot be seen — the ultra-microscopic viruses. Dark field illumina- 

 tion has done much to facilitate the study of these forms, but, as yet, 

 the exact nature of the numerous minute objects which it has revealed 

 in every fluid, and which are in constant motion (Brownian movement), 

 has not been satisfactorily determined, so that at present it is in many 

 cases impossible to decide whether they are actually living organisms or 

 granules of inanimate material. 



The study of microscopic organisms has revealed the fact that, in 

 their method of nutrition, some of them resemble plants and others 

 animals. On the basis of this physiological distinction it has been the 

 custom to regard them as belonging to one of two main groups — the 

 Protophyta and the Protozoa. The study of the former has been relegated 

 to the botanist, and that of the latter to the zoologist. Though some of 

 these organisms show undoubted affinities with the algse and higher 

 plants and others with animals, there exists a miscellaneous assemblage 

 of indeterminate forms which cannot be placed legitimately in either 

 group. Accordingly, it is safer to regard them all as belonging to one 

 large group, the Protista, the study of which is known as Protistology, 

 as first suggested by Haeckel (1866). Without being able to define 

 accurately the limits of either group, it is nevertheless convenient to 

 regard the Protista as comprising the two subdivisions of the Protozoa 

 and the Protophyta. In the case of the former, nutrition is effected by 

 the ingestion of preformed proteid material, either as solid particles or 

 in solution. The Protophyta, on the other hand, nourish themselves 

 like plants on comparatively simple chemical compounds, and when 

 possessing chlorophyll or some similar substance, make use of the carbonic 

 acid of the liquid in which they live. Very frequently they secrete around 

 themselves capsules of cellulose. A typical Protist consists of a small 

 portion of cytoplasm and a nucleus which contains as its most essential 

 constituent a substance called chromatin. The contents of the nucleus 

 are separated from the cytoplasm by a nuclear membrane. Other bodies 

 may be present in the cytoplasm, but these, at least as definite visible 

 structures, are not essential to life. 



Amongst the existing Protista the most primitive forms are possibly 

 the bacteria, spirochsetes, and allied organisms, which in many cases do 

 not appear to possess definitely constituted nuclei, though granules of 

 a substance which some observers have identified with chromatin are 

 present in the cytoplasm. Alexeieff (1924fl) maintains that it is not 

 chromatin, and that this substaiice is absent from bacteria. These 

 forms, however, are in most cases so minute that accurate information 

 regarding their cytological structure and life-histories is difficult to obtain. 

 It can, at any rate, be safely affirmed that those Protista which are most 



