n 



PHYLUM PROTOZOA 



91 



r.Td 



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 micronucleiis (pa. nu.), is a very small body 

 closely applied 



to the mega- ^pm* B 



nucleus ; when 

 it divides it 

 goes through 

 the complex 

 series of stages 

 characteristic of 

 mitosis (p. 18). 

 The contrac- 

 tile vacuoles (c. 

 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. 

 buccal groove ; corf, cortex ; CM. cuticle ; c. vac. contractile 

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

 meganucleus ; pa. nu. micronucleus ; trch. trichocysts. (From 

 Parker's Biology.) 



