THE GENERAL PHYSIOLOGY OF THE PROTOZOA 187 



1. Nutrition and Assimilation. Living organisms, considered 

 generally, exhibit a great variety of methods of nutrition, which 

 may be classified into two main groups ; bearing in mind, however, 

 that in all classifications of living beings, or of their vital properties, 

 any groups or classes that can be distinguished are always connected 

 by gradual and imperceptible transitions, and that consequently 

 forms will present themselves which, owing either to their transi- 

 tional nature or to the imperfect state of our knowledge concerning 

 them, can only be assigned to one or the other group in a manner 

 as arbitrary as the statement that the 21st of June is the first day 

 of summer a difficulty which in no way invalidates the distinction 

 between spring and summer. . 



In the first place, many organisms can build up the complex 

 protein-substances, of which the living protoplasm is composed, 

 from simpler chemical materials. Of this type there are found 

 among Protozoa, as already stated, two types of nutrition : first, the 

 holophytic, or plant-like, in which the organism is able, by means of 

 special cell-organs, to utilize the energy of the sunlight in order to 

 synthesize its body-substance from the simplest chemical materials, 

 such as water, carbon dioxide, and mineral salts, through a series 

 of substances in an ascending scale of chemical complexity ; 

 secondly, the saprophytic type, in which the body contains no visible 

 organs subserving the function of nutrition, but the organism is 

 able to build up its protoplasm from food-materials consisting of 

 organic substances in solution which are far less complex chemically 

 than the body-proteins. 



In the second place, many organisms cannot build up their body- 

 substance from materials of simpler chemical constitution, but are 

 entirely dependent on a supply of protein-substance ready-made, 

 which they obtain either by ingesting and digesting other living 

 organisms in the holozoic method, or by living as parasites at the 

 expense of other creatures. These two methods graduate into one 

 another, since many parasites simply devour portions of the bodies 

 of their hosts in a holozoic manner, but the majority of parasites 

 absorb fluid nutriment from their hosts in an osmotic manner ; 

 hence it is convenient to distinguish holozoic and osmotic parasites. 



Considering these various methods of nutrition, it is seen that, 

 from the point of view of the nature of the food, those which ingest 

 solid food-particles (holozoic forms) can be distinguished from those 

 which absorb their food in a diffused or dissolved condition (holo- 

 phytic and saprophytic forms and osmotic parasites). From the 

 point of view of the structure of the organism, those which possess 

 special organs of nutrition (holozoic and holophytic forms) can be 

 distinguished from those which possess none (saprophytic forms and 

 osmotic parasites). 



