Fig. 18. — Paramoecium caudatum. A, the living animal from ihe 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; cort, cortex; cu, cuticle: c. vac, contractile vacuole; f. 

 vac food vacuole; gul, gullet; med, medulla; nu, meganucleus; pa. nu , micro- 

 nucleus; trch, trichocysts. (From Parker's Biology.') 



47 



