INTRODUCTION 



31 



a tendency to become globular and this is the form assumed by 

 the great majority of Protozoa when they encyst. The spherical 

 or homaxonic type, furthermore, is characteristic, not only of free 

 floating forms, but also of the most generalized representatives of 

 all classes of Protozoa. 



While density or consistency of the protoplasm is thus one of the 

 factors determining form in Protozoa, its effect in the majority of 

 types is offset by the presence of definite membranes, shells, tests 

 and skeletons; by specialized protoplasmic differentiations; or by 

 foreign bodies. Thus the density of the sluggish Pelomyxa palustris 



Fig. 9. — Euglypha alveolala (A), and Cochlio podium, sp. (B). (After Calkins.) 



is due to the enormous number of crystals of mud and sand, shells 

 of diatoms and peculiar refractile bodies resembling glycogen in 

 make up. Membranes of living substance, as in Cochlioyodium 

 (Fig. 9) and the majority of flagellates and ciliates, of lifeless chitin 

 as in Allogromia oviforme (Fig. 10) or the lifeless materials secreted 

 by the cell and deposited on it are responsible for the forms assumed 

 by many Protozoa. Even delicate types such as Clathrvlina elegans 

 and the majority of Heliozoa retain their forms by virtue of the 

 protecting shells of lifeless materials deposited on a chitinous mem- 

 brane. The protoplasmic bodies of many of the fresh water shelled 



