THE SIMPLER ORGANISMS 79 



3. The phenomena of division, in a series of outhne 

 sketches. 

 Vorticella. — This protozoan will usually be found associa- 

 ted with Stentor and specimens for study are readily ob- 

 tained by the same means. The individuals are smaller, 

 and singly are difficult to see ; but they commonly occur in 

 groups, and a little cluster of them about a twig, contracting 

 so strongly as to almost disappear when touched, will be 

 easily recognized . Vorticellas when abundant appear to the 

 unaided eye as a whitish fringe about the edges of submerged 

 twigs. The student should obtain upon a slide a small bit 

 of rootlet or other solid support with vorticellas attached, 

 should mount and cover this, filling "up vrith vvater all the 

 space beneath the cover, and then should perform the work 

 of the following outline : 



1 , Survey the mount for : 



a) Single vorticellas contracting and extending their 

 stalks. 



b) Little groups of individuals, attached to the rootlet 

 separately. 



c) Detached heads, broken oft: from the stalks and 

 swimming free. 



2. Study the actions of the vorticellas, observing: 



a) The contraction and subsequent extension of the 

 stalk. 



b) The closing and opening of the peristome. 



c) The action of the cilia and its effects on free particles 

 in the water. 



d) The action of the vorticellas toward one another 

 when touched, and toward other free swimming 

 organisms which happen to come into contact with 

 them. 



3 Study the differentiation of pares in the body of Vorti- 

 cella, noting: 



