CHAP, m VORTICELLA 271 



observation. There is a meganucleus (nn) remarkable 

 for its elongated band-like form, and having in its neigh- 

 bourhood a minute micronucleus. Cilia are also present, 

 but the way in which they are disposed is very peculiar 

 and characteristic. To understand it we must study the 

 form of the body a little 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 peristome is 

 a depression, the mouth (mth], leading into a conical 

 gullet (gull). 



There Is reason for thinking that the whole proximal 

 region of Vorticella answers to the ventral surface of Para- 

 moecium, 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 speci- 

 men 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 bend- 

 ing down of successive blades of grass produces a series 



