18 GENERAL ORGANIZATION OF THE PROTOZOA 



manifold varieties of protozoon structure. To describe all of these 

 modifications under one or a few headings, and to attempt to formulate 

 general laws from the different and often highly complicated life 

 histories, is out of the question. Nevertheless, in spite of the struc- 

 tural modifications and special adaptations to particular modes of 

 life, it is possible to group the different kinds of protozoa in four defi- 

 nite types, first outlined by the French microscopist Felix Dujardin 

 in 1841. Three of these types — sarcodina, mastigophora, and infu- 

 soria — are based upon the form of the locomotor organs, pseudopodia, 

 flagella, and cilia respectively, while the fourth type — sporozoa — 

 including the gregarinida, first recognized as unicellular organisms 

 by Kolliker in 1845, are devoid of motile organs, and are invariably 

 parasitic in mode of life (Fig. 1). 



A. GENERAL MORPHOLOGY. 



While the different kinds of protozoa are undoubtedly the simplest 

 animals known to us, they comprise at the same time some of the most 

 complicated forms of cells, and the protoplasmic differentiations within 

 these cells are frequently highly developed. In some cases these 

 modifications are so highly evolved that we have little reason to regard 

 such cells as units of structure comparable with the tissue cells of 

 higher animals and plants, but should look upon them as composed 

 of still more elementary vital units, and to this extent the cell theory, 

 when applied to them, is inadequate. 



The wide distribution of the protozoa and their varied modes of 

 life lead to the greatest possible differences between them and even 

 within the limits of the same class. No one form is characteristic of 

 any type, but in all cases where the body is plastic and subjected to an 

 even environmental pressure, as in floating, or in intracellular, quies- 

 cent forms, the body is spherical (homaxonic), readily changing, how- 

 ever, into an elongate or monaxonic form when the organism moves 

 or is subjected to a current. In all divisions, when for any reason the 

 surrounding medium becomes unsuitable, or in some cases for pur- 

 poses of digestion or reproduction, the organisms secrete a thick and 

 resistant covering of chitin, and they remain thus "encysted" until 

 conditions are again suitable, and such cysts are usually spherical. 



The size of protozoa likewise varies within wide limits. Some of 

 them are on the very limits of vision, and some, apparently, are 

 invisible, even when the eyes are assisted by the highest powers of the 

 microscope. Thus, the organism causing yellow fever, and thought to 

 be a protozoon, is so minute that it has never been seen, although its 

 habitat and its general history are well known. Other protozoa, on the 

 other hand, are relatively enormous single cells, a Pdomyxa palusiris 



