ORGANELLES 



29 



divisible into an outer and inner zone, the ectoplasm and endo- 

 plasm, respectively (Fig. 1). There is no fundamental difference 

 between these two layers of protoplasm, merely a difference in 

 density. The ectoplasm is the less fluid and comparatively clear, 

 while the endoplasm is more fluid and somewhat granular. The 

 clearness of the differentiation between ectoplasm and endo- 

 plasm is sometimes useful in distinguishing species of protozoans, 

 especially amebae. The ectoplasm differs from the endoplasm 



Fig. 2. Tj'pes of organs of locomotion in Protozoa; A, Amahn with psfiudo- 

 podia; 5, a heliozoan with "axopodia"; C, 5o<io with free flagella ; D, Trypanosoma 

 with flagellum attached to undulating membrane; E, Choanofiagellato with flagcl- 

 lum and "collar"; F, Plvuronemu with cilia and undulating membrane formed of 

 fused cilia; G, modes of insertion of cilia; H, Aspidisca with cirri. (Figs. /'' to H 

 from Calkins.) 



in function as well as in appearance. The ectoplasm may be 

 likened to the body wall and appendages of higher animals whik; 

 the endoplasm may be compared with the viscera or inter- 

 nal organs. The endoplasm digests food and has the power of 

 secretion and excretion, while the ectoplasm produces the vari- 

 ous organelles for locomotion, food getting, oxygen absorption 

 and special senses. The term " organelle " is used in place of 

 " organ " for structures which are only parts of a single cell. 



Organelles. — The organelles contained in a protozoan's body 

 may be many and varied. Those connected with movement or 

 locomotion differ in different groups and form the chief charac- 

 teristic on which the usual classification into Sarcodina, Flagellata, 



