PROTOZOA. 1 5 



ciliary current brings dissolved oxygen with it, and also carries 

 away waste-products. 



3. Reproduction (Fig. 2). This is asexual, and usually takes 

 place by equal binary fission in a longitudinal direction. The 

 animal broadens, its nucleus elongating, and then a furrow appears, 

 which rapidly deepens so that the animal is cleft down to its 

 stalk, the nucleus and contractile vacuole being halved. Two 

 equal-sized individuals result, one of which remains attached to 

 the stalk, while the other develops a circlet of cilia near its prox- 

 imal end, becomes detached, and swims away, later on becoming 

 fixed by its proximal end and developing a stalk. Thus Vorticella, 

 although a fixed form, can readily spread from place to place, and 

 it is, therefore, not to be wondered at that it has a very wide dis- 

 tribution, especially if it be remembered that the process of fission 

 only takes an hour or two for its completion. 



Small free-swimming individuals (microzooids), similar in struc- 

 ture to the large free forms, may be produced by unequal fission, 

 in which case an ordinary zooid divides into two parts, one large, 

 the other small, or else continued fission may take place in 

 which equal bipartition is immediately followed by rapid division 

 of the half to be detached into eight microzooids. Using large 

 letters for the larger zooids (macrozooids) and small ones for the 

 microzooids, the different kinds of fission may be expressed as 

 follows : 



Equal Fission. Unequal Fission. 



B (fixec 

 B' (free) 



Continued Fission. 



^ |B (fixed) A _^ /B (fixed) 



C C C C 



A process, known as conjugation, which has an important 

 bearing on the origin of sexual reproduction, also frequently 

 occurs in Vorticella. A microzooid comes into contact, by its 

 ciliated end, with a large fixed macrozooid, at a point near the 

 junction of the stalk. The two gradually fuse together, parts of the 



