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. Types of organs of locomotion in Protozoa; A, Amceba with pseudo- 

 podia; B, a heliozoan with "axopodia"; C, Bodo with free flagella; D, Trypanosoma 

 with flagellum attached to undulating membrane; E, Choanoflagellate with flagel- 

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

 fused cilia; G, modes of insertion of cilia; H, Aspidisca with cirri. (Figs. F 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 while 

 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, 



