INTRODUCTION 31 



(2) Membranes, Shells and Skeletons as Form-determining Factors.— 

 While density or consistency oi 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, 



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 1 ^Hlll 



V, \^ \ 



r- I 





h : ^ ,: 



%. ^ ^*^- » -5- ■ --1* • - " A JP ' 



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yf .-■ 



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Fig. 9. — Allogromia oviforme, foramiiiiferon with chitinous monothalamous shell 

 and reticulose pseudopodia. (D) a recently captured diatom; (S) chitinous shell. 

 (From Calkins after M. Schultze.) 



and skeletons; by specialized protoplasmic differentiations; or by 

 foreign bodies. Thus the density of the sluggish Pclomyxa pulustris 

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

 of diatoms and peculiar refractile bodies resembling glycogen in 



