530 GENERAL CONSIDERATION OF PROTOZOA 



the morphology and the biologic characters of a very limited number 

 of families. 



Shape and Size. Protozoa, composed as they are of a soft proto- 

 plasm, present themselves under various shapes, depending upon 

 differences in environment; they may be more or less spherical, or 

 homaxonic, or they may be decidedly elongated in one axis, or mon- 

 axonic. When the environments become unfavorable and under 

 other conditions, protozoa often become perfectly spherical, and 

 secrete a thick, resistant membrane composed of chitin. This change 

 is known as the encysted stage. The organism may die in it or it may 

 relinquish the encysted condition and return to its former shape, 

 divide under the protecting membrane into daughter cells, or sporulate 

 when circumstances become more favorable. Protozoa are evidently 

 more resistant in the encysted stage, and it may in certain respects 

 be likened to the spore formation in bacteria, though spore formation 

 in protozoa is a process quite different and distinct from encysting. 

 As a class of organisms protozoa vary so much in shape that no 

 common description will fit all of them. They likewise vary greatly 

 in size (from one or a few micra to several millimeters), and it cannot 

 even be said that all protozoa are microorganisms, as some can be 

 easily seen with the naked eye. Porospora gigantea, a gregarina 

 found in the intestines of the lobster, is 16 mm. long. The individual 

 organisms of one and the same species will also frequently vary 

 considerably in size, much more than bacteria. This variability 

 often depends upon the amount of available food supply and upon 

 other external conditions. 



Structure. Bacteria do not yet show a differentiation of the cell 

 into a protoplasmic body and a nucleus ; all protozoa, however, possess 

 a distinct nucleus or several nuclei. The cell protoplasm or cytoplasm 

 of protozoa is composed of the spongioplasm, which generally has a 

 network or honey-combed structure, and is made up of a rather firm, 

 tenacious substance, and the hyaloplasm, which is a more fluid 

 substance contained in and filling out the network of the spongio- 

 plasm. The outer layer of the protozoan organism generally is 

 composed of a more condensed and tougher protoplasm called the 

 ectoplasm in contradistinction to the softer protoplasm within known 

 as the entoplasm. The ectoplasm sometimes secretes or forms a 

 membrane, an armor, organs of locomotion, etc. Round or oval 

 spaces called vacuoles are frequently seen inside of the protoplasm. 

 They are not empty or air-containing spaces, but are filled with a 

 watery fluid and are either concerned in the digestion and assimilation 

 of food particles or are contractile vacuoles which empty and refill. 

 Protozoa sometimes contain a complicated system of intercommuni- 

 cating vacuoles through which fluid more or less constantly circulates. 

 In addition to the vacuoles the cytoplasm of protozoa shows granules 

 of various size and shape. Thus the small granules of Altmann, 

 supposed to be intimately connected with the ultimate structure and 



