PHYLUM PROTOZOA 



89 



protoplasm: they then contract, discharging the water into the 

 vacuole, the latter rapidly enlarging while they disappear from 



nit. 



c.rac 



Fig. 69. — Paraxnoeciuxn caudaf um. 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 

 wliich has discharged its trichocysts ; D, diagram of binary fission ; &wc. gr. buccal groove ; 

 cort. cortex ; rv,. cuticle ; c. vac. contractile vacuole ; ,;'. vac. food vacuole ; gul. gullet ; ^iied. 

 medulla; ii?(.meganucleus; 25«- ?iw. micronucleus ; (*■€/(. trichocysts. (From 'Purker'a Molog]/.) 



view ; finally the vacuole contracts and discharges its contents 

 externally. 



The cortex contains minute radially arranged sacs called 

 trichocysts {trch.). When the animal is irritated, more or fewer of 



