GENERAL SKETCH 



53 



of Protozoa there is no known organ by which the waste products are 

 removed. In such forms excretion probably takes place by osmosis 

 through the walls of the body, in the same way possibly that sapro- 

 phytic forms take food. This must be the case in the Sporozoa and 

 many marine forms as well as in certain flagellates, in which there is 

 no specialized excretory organ. In the majority of the Protozoa, 

 however, there are specialized structures which regularly throw to the 

 outside of the organism a certain amount of fluid substance. These 

 structures are the contractile 

 vacnoles, which, with the ex- 

 ceptions of the Sporozoa and 

 the marine forms, are found 

 in every class of the Protozoa. 

 In the living animal the vacu- 

 ole is a clear spherical area in 

 the endoplasm. It is formed 

 by the slow addition of water 

 from the endoplasm, and grows 

 until a maximum size is reached, 

 when it suddenly disappears, ty_ 

 the contained water being 

 driven to the outside. Vacu- 

 oles are frequently variable in 

 position (Sarcodina), while the C ~ 

 number is, to a certain extent, 

 dependent upon the condition 

 of the protoplasm, several ob- 

 servers having shown that, as 

 the individuals lose their vital- 

 ity, the protoplasm becomes 

 more and more vacuolated. In , 



Fig. 21. Ft -ontoma, leucas Ehr. [SCHEWIAKOFF.J 



many Cases the VaCUOle moves c> canal . v> vacuole with external pore ; N, macro- 

 about With the endoplasmk nucleus ;, micronucleus. 



flow until, becoming heavier than the protoplasm, it remains stationary, 

 while the rest of the endoplasm moves forward with the organism 

 (many Rhizopoda). In this manner the vacuole, as it attains its full 

 size, is gradually left at the posterior end of the moving organism, 

 where it finally bursts. Again, as in some Mastigophora and Ciliata, 

 the contractile vacuole is a stationary organ connected with the out- 

 side by a definite pore. Here, too, are numerous accessory structures 

 in the form of canals and reservoirs, the former apparently collecting 

 the water and waste matters from all parts of the cell, and conducting 

 them to the contractile vesicle, the latter receiving the fluid after con- 

 traction of the vacuole, and conveying it to the outside, with which 



