34 HANDBOOK OF PROTOZOOLOGY 



macronucleus, becoming fully formed during the course of their 

 migration toward the periphery of the body. The extrusion of 

 trichocysts is easily induced by means of mechanical pressure 

 or chemical stimulation, though the mechanism of the extrusion 

 is not well understood. Brodsky maintains that the fundamen- 

 tal force is not the mechanical pressure, but that the expansion 

 of the colloidal substances results under certain stimuli in the 

 extrusion of the trichocysts through the pellicle. The fully 

 extruded trichocyst is an elongated structure with a drawn-out 

 free end and with a small cap-like structure and a fine filament at 

 its base. The fully extruded trichocysts of Paramecium cauda- 

 tum are as much as 40 microns in length. 



In certain ciliates there occur rod-like trichites which or- 

 dinarily surround the cytopharynx (Fig. 13). Reserve trichites 

 are scattered throughout the endoplasm. They apparently serve 

 to strengthen the cytostome and cytopharynx and to capture 

 and hold actively motile Protozoa on which the organisms live. 

 In the spores of Cnidosporidia there is a characteristic filament- 

 ous structure known as the polar filament, which is spirally 

 coiled in an envelope, the polar capsule (Fig. 133). Under a 

 suitable stimulation, the filament extrudes and serves probably 

 for temporary anchoring of the spore to the gut-epithelium of 

 the host. A similar structure is also found in some Dinoflagel- 

 lida. 



In the pellicle-bearing Protozoa there is always a definite 

 cytostome which varies considerably in size, form and location 

 in different forms. In those that feed continuously, such as Para- 

 mecium, the cytostome is permanently open, and in others, it is 

 closed except at the time of food-taking. These forms also have 

 a cytopyge, or cell-anus, through which undigested solid particles 

 or waste bodies are extruded to the exterior. It is usually closed 

 and difficult of observation except during the actual defecation 

 (Fig. 14). 



The cytostome leads into a tubular widening of the endo- 

 plasm, known as the cytopharynx, or gullet. The cytopharynx 

 may possess an undulating membrane or cilia. It may be sur- 

 rounded by trichites as has been stated. At the end of the cyto- 

 pharynx the food vacuole is first formed. 



The mode of nutrition is considered as an important means 



