ra PRACTICAL DIRECTIONS 278 



5. The structure of the cortex, which is similar to that of 

 Paramoecium, except that the cilia have a restricted dis- 

 tribution, and 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. 

 The contractile vacitole is single. 



6. The medulla, which circulates and contains food- 

 vacuoles, as in Paramoecium. In it are situated an elongated 

 and curved meganucleus and a minute micronucleus, which 

 is not easy to distinguish. Sketch. 



7. Make preparations as directed under Paramcecium. 

 Sketch. 



8. Look out for specimens undergoing fission, noting the 

 different stages, and the second, proximal 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. 



Carchesium or Epistylis. You will very likely find 

 specimens of one of these, or of an allied genus, amongst the 

 Vorticellae. Xote that several individual zooids are borne 

 upon a branched stalk, together forming a colony. 



