8 THE PEOTOZOA 



Both freshwater and marine forms of Vorticella are known. 

 The following description applies more particularly to some 

 of the larger freshwater species, but most of the marine forms 

 closely resemble these. 



Mount a specimen in a drop of water together with a hit 

 of the weed or other substance to which it is attached. Examine 

 it mdcroscopically. 



A. General appearance. 



1. The form of the fully expanded animal is bell-shaped, the 



mouth of the bell being closed by the circular disc, 

 and the handle of the bell represented by the slender 

 stalk by which the animal is attached. The disc is 

 placed somewhat eccentrically : it is in contact with 

 the edge of the bell at the ' dorsal ' surface, but sepa- 

 rated from it by a groove at the opposite or ' ventral' 

 edge. 



2. The peristome is the thickened everted margin of the 



bell. It bears a fringe of cilia. 



3. The margin of the disc bears a fringe of cilia, uniting 



at each side with the peristomial fringe mentioned 

 above. 



4. The vestibule is a conical cavity opening exteriorly 



between the ventral margins of the disc and peri- 

 stome. The cUia with which it is provided cause 

 a rapid current of water to flow into and out of the 

 vestibule. 



If this current be not visible, run under the cover-glass a 

 little water containing some insoluble pigment, e.g. carmine 

 or indigo. Indicate the direction of the cwrrent in the vesti- 

 bule by a/rrows on your drawing. 



5. The pharynx is the narrower continuation of the vesti- 



bule inwards. Its cilia drive water, bearing food- 

 particles, into the endosarc. 



6. The anus opens into the vestibule, but is only visible at 



the moment of extrusion of faecal matter. 



7. The contractility of the animal is exhibited when it is 



