IT 



PHYLUM PROTOZOA 



91 



Tntc.e/7: 



v.vac. 



is a large ovoid body staining evenly with aniline dyes, which, 

 when it divides, does so directly by a simple process of constriction* 

 The other, called the micronucleus (pa. nu.), is a very small body 

 closely applied 

 to the mega- 

 nucleus ; when 

 it divide's it 

 goes through 

 the complex 

 series of stages 

 characteristic of 

 mitosis (p. 18). 

 The contrac- 

 tile vacuoles (c. 



-"> "& ~ l- .^ S* ' V /TV+'I 



vac.) are two in 

 number, and 

 are very readily 

 made out. 

 Each is con- 

 nected with a 

 series of radia- 

 ting spindle- 

 shaped cavities 

 in the proto- 

 plasm which 

 serve as feeders 

 to it. After 

 the contraction 

 of the vacuole 

 these cavities 

 are seen gradu- 

 ally to fill, 

 apparently re- 

 ceiving water 

 from the sur- 

 rounding proto- 

 plasm : they 

 then contract, 

 discharging the 

 water into the 

 vacuole, the 

 latter rapidly 

 enlarging while 

 they disappear 

 from view; 



finally the vacuole contracts and discharges its contents externally. 

 The cortex contains minute radially arranged sacs called 



nu. 



FIG. 70. Paramoecium caudatum. A, the living animal from 

 the ventral aspect ; B, the same in optical section : the arrow 

 shows the course taken by food-particles ; C, a specimen which has 

 discharged its trichocysts ; D, diagram of binary fission ; buc. gr, 

 buccaJ groove ; cort. cortex ; CM. cuticle ; c. vac. contractile 

 vacuole : /. vac. food vacuole ; gul. gullet ; med. medulla nu. 

 meganucleus ; pa. nu. micronucleus ; trek, trichocysts. (From 

 Parker's Biology.) 



