30 BIOLOGY OF THE PROTOZOA 



individuals one-sixteenth the original size (Fig. 200, p. 485). To 

 the uninitiated such variations in forms and habits offer great temp- 

 tation to swell the list of synonyms. 



A. Form-relations of Protozoa. The forms of Protozoa are highly 

 varied and depend to some extent upon the mode of life, to some 

 extent upon the mode of reproduction and to some extent upon 

 their lifeless skeleton elements, but in the last analysis they depend 

 upon the physical consistency of the protoplasm. Fluid types, if 

 not confined by resistant cell membranes, readily change in form 

 according to environmental conditions, or by virtue of forces coming 

 from metabolic activities within. Amoeba proteus and other species 

 of Ameba are amorphous and are constantly changing in shape, a 

 characteristic phenomenon to which the term ameboid movement 

 is applied, and the same protoplasm may be spherical in form, or 

 flattened on the substratum, or extended in various ways. Many 

 forms, under certain pressure conditions in the surrounding medium 

 due to evaporation or reduced volume of water, will suddenly burst 

 and disappear leaving no trace whatsoever of their previous presence. 

 This phenomenon has been repeatedly mentioned by earlier observ- 

 ers in connection with types of Protozoa belonging to all classes, 

 and the term diffluence was applied to it by Dujardin. In such cases 

 the fluid protoplasm is usually confined by a resisting membrane 

 or cortex which remains intact during the ordinary phases of activ- 

 ity, but when the pressure from within becomes too great for the 

 resistance of the membrane the latter collapses, the cell disappear- 

 ing with all the characteristics of a miniature explosion. 



Another evidence of the difference in density between different 

 species of Protozoa is the reaction after cutting with a scalpel. 

 Some species, for example Paramecium cavdatum, are extremely 

 difficult to cut successfully owing to the fluid character of the inner 

 protoplasm which, as soon as the cortex is cut, flows out and disin- 

 tegrates; in my experience not more than 20 per cent out of more 

 than 1000 operations on Paramecium caudatum have been success- 

 ful, but the percentage is greatly increased by preliminary treat- 

 ment with neutral red. Other forms of ciliates on the other hand 

 may be cut in any plane, Uronychia transfuga and Uroleptus mobilis 

 for example, reacting to such operations with all the physical 

 properties of a piece of cheese. 



The more fluid Protozoa, when the form is not maintained by 

 resistant cortical differentiations, react to physical properties of 

 the surrounding medium. When forces on all sides are equal, as in 

 suspended water-dwelling types like Actinophrys sol, Actinosphae- 

 rium, many Radiolaria, etc., the form is spherical, or spherical also 

 in parasitic forms enclosed in the protoplasm of the host cell as is 

 the case with the majority of Coccidia. In all types, under certain 

 environmental conditions, or when continuously irritated, there is 



