VORTICELLA. 15 



3. The mouth of the bell is nearly filled by a struc- 

 ture (Fig. 4, b), which may be compared to the clapper 

 of a bell, and which is known as the ciliated disc or e/>/x- 

 tome. Its outer or upper surface is slightly arched ; and 

 there is a second circlet of long cilia (Fig. 4, a) around 

 its edge. 



4. Around the greater part of its circumference the cil- 

 iated disc is united to the peristome ; but on one side there 

 is an open space, the vestibule (Fig. 4, d), which is 

 bounded internally by the disc, and externally by the 

 peristome. 



5. Notice that, when the animal is fully expanded, the 

 plane of the peristome makes an acute angle with the 

 plane of the ciliated disc : the vertex being opposite the 

 vestibule. 



6. In the lower part of the bell notice a number of faint 

 longitudinal striations which may, in favorable specimens, 

 be seen to cover the whole surface of the bell up to 

 the peristome. 



b. The stem is cylindrical, and consists of an outer, 

 transparent sheath (Fig. 4, I) and a central, darker axis 

 (Fig. 4, wi), which is not straight, but arranged in a 

 loose spiral inside the tube formed by the outer sheath. 



c. Make a sketch showing these points. 



VII. Selecting an individual with a short stem, watch 

 the process of contraction, and notice the following 

 changes : 



a. The ciliated disc is first withdrawn into the bell by 

 a process of rotation upon the peristome at a point oppo- 

 site the vestibule. 



b. The cilia of the peristome cease vibrating and fold 

 in over the disc. 



c. The peristome next folds inwards and contracts, and 

 the body becomes nearly spherical. 



