CHAP. Ill VORTICELLA 275 



they are disposed is very peculiar and characteristic. To 

 understand it we must study the form of the body a Httle 

 more closely. 



The conical body is attached by its apex or proximal 

 end to the stalk : its base or distal end is expanded 

 so as to form a thickened rim, the peristome {per), within 

 which is a plate-like body elevated on one side, called 

 the disc (d), and looking like the partly raised lid of a 

 chalice. Between the raised side of the disc and the peri- 

 stome is a depression, the mouth (inth), leading into a 

 conical gullet (gull). 



There is reason for thinking that the whole proximal region 

 of Vorticella answers to the ventral surface of Paramoecium, 

 and its distal surface with the peristome and disc to the 

 dorsal surface of the free-swimming genus : the mouth is to 

 the left in both. 



A single row of cilia is disposed round the inner border 

 of the peristome and continued on the one hand down the 

 gullet, and on the other round the elevated portion of the 

 disc ; the whole row of cilia thus takes a spiral direction 

 The rest of the body is completely bare of cilia. 



The movements of the cilia produce a very curious optical 

 illusion : as one watches a fully-expanded specimen it is 

 hardly possible to believe that the peristome and disc are 

 not actually revolving — a state of things which would imply 

 that they were discontinuous from the rest of the body. As 

 a matter of fact the appearance is due to the successive 

 contraction of all the cilia in the same direction, and is 

 analogous to that produced by a strong Wind on a field of 

 corn or long grass. The bending down of successive 

 blades of grass produces a series of waves travelling across 

 the field in the direction of the wind. If instead of a field 

 we had a large circle of grass, and if this were acted upon 



T 2 



