THE SIMPLER ORGANISMS 



81 



3. An outline drawing illustrating the manner of dividing. 

 Colonial Vorticellidae. In a number of protozoans 

 allied to Vorticella, the two cells resulting from a 

 division do not entirely separate, 

 but both remain attached basally 

 to the common stalk, each later 

 prolonging the attachment into a 

 stalk of its own. Successive divi- 

 sions thus give rise to colonies. Such 

 colonies are likely to be found asso- 

 ciated with Vorticella, and should 

 be compared therewith. When the 

 colonies are large they are easily 

 distinguished with the unaided eye 

 from clusters of Vorticella by their 

 height, due to their elevation on a 

 common stalk. One of the com- 

 monest of these is Epistylis, dia- 

 grammatically shown in the accom- 

 panying figure (fig. 54). This 

 differs from Vorticella in that the 

 stalk is not contractile, lacking 

 Fic.54. Epistylis umbeiiarius. the myoneme : myonemes are re- 

 stricted to the base of the elong- 

 ated "head" which, becomes trans- 

 versely wrinkled when contracted, 

 and tothe peristome which becomes 

 enrolled, as in Vorticella. 



^ Charchesium, however, 

 the individual stalks are con- 

 tractile and in Zoothamnium, the common stalk of the 

 colony also, in-so-much that when Zoothamnium contracts, 

 the main stalk and all its branches acting synchron- 

 ously, all the bodies are suddenly brought down into a 





