PARAMECIUM 



the trichocysts appear as long threads which have been extended 

 to about eight times their former length (Fig. 24, C). Tricho- 

 cysts are supposed to function as weapons of offense and defense. 

 It is said that their contents are discharged with considerable 

 force and that they contain a poison strong enough to paralyze 

 any single-celled animal. The only evidence we have that the 

 trichocysts are weapons of defense is furnished when Paramecium 

 encounters its enemy Didinium. If the seizing organ of this 

 Protozoon becomes fastened in the Paramecium, a great number 

 of trichocysts near the place of the injury are discharged (Fig. 26). 

 These produce a substance which be- 

 comes jelly-like on entering the water; 

 this tends to force the two animals apart, 

 and, if the Paramecium 

 is a large one, frequently 

 it succeeds in making 

 its escape (70). 



FIG. 26. Paramecium defending itself from an attack by a Protozoon 

 Didinium. The trichocysts are discharged and mechanically force the 

 enemy away. (From Mast in Biol. Bui.) 



Two contractile vacuoles are present, occupying definite posi- 

 tions, one near either end of the body. They lie between the 

 ectoplasm and the endoplasm, close to the dorsal surface, and com- 

 municate with a large portion of the body by means of a system 

 of radiating canals, six to ten in number. The vacuoles grow in 

 size by the addition of liquid which is excreted by the protoplasm 

 into the canals and is then poured into them. When the full 



