288 PRACTICAL DIRFXTIONS CHAP. Ill 



there are no trichocysts. Moreover, in the stalk (into which the 

 medulla does not extend) the deeper layer of the cortex gives rise to a 

 central contractile axial fibre, by means of which the stalk can contract 

 spirally. 



6. The medulla, which circulates and contains food-vaciioles, as in 

 Paramoecium. The single spherical contractile vacuole is also situated 

 in it, as well as an elongated and curved megamicleiis and a small 

 iiiicronucleus (not easy to distinguish). Sketch. 



7. Make preparations as directed under Paramoecium. Sketch. 



8. Look out for specimens undergoing fission, noting the different 

 stages, and the second, aboral ring of cilia on one of the daughter 

 individuals, which eventually becomes detached. Note these free 

 swimming forms, and also search for conjugating individuals — a small 

 free Vorticella uniting with a large stationary one. Sketch. 



Carclxesium or Epistylis. — You will very likely find specimens of 

 one of these, or of an allied genus, amongst the Vorticellse. Note 

 that several individual zooids are borne upon a branched stalk, together 

 forming a colony. 



