132 VORTICELLA AND ZOOTHAMNIUM less. 



the numbers of the species in a given spot that the food- 

 supply would inevitably run short. This is prevented by 

 one of the two sister-cells produced by fission leading a free 

 existence long enough to enable it to emigrate and settle in 

 a new locality, where the competition with its fellows will be 

 less keen. The production of these free-swimming zooids 

 is therefore a means of dispersal (see p. 124) : contrivances 

 having this object in view are a very general characteristic 

 of fixed as of parasitic organisms. 



Conjugation occasionally takes place, and presents certain 

 peculiarities. A Vorticella divides either into two unequal 

 parts (f^) or into two equal halves, one of which divides 

 again into from two to eight daughter-cells (f^). There are 

 thus produced from one to eight microzooids which resemble 

 the barrel-shaped form (k^) in all but size, and like it become 

 detached and swim freely by means of a basal circlet of cilia. 

 After swimming about for a time, one of these microzooids 

 comes in contact with an ordinary form or megazooid, when 

 it attaches itself to it near the proximal end (g^), and under 

 goes gradual absorption (g^), the mega- and microzooids 

 becoming completely and permanently fused. As in Para- 

 moecium, conjugation is followed by increased activity in 

 feeding and dividing (p. 116). 



Notice that in this case the conjugating bodies or gametes 

 are not of equal size and similar characters, but one, which 

 is conveniently distinguished as the microgamete ( = micro- 

 zooid) is relatively small and active, while the other or 

 megagaftiete ( = megazooid, or ordinary individual) is rela- 

 tively large and passive. As we shall see in a later lesson, 

 this differentiation of the gametes is precisely what we get in 

 almost all organisms with two sexes : the microgamete being 

 the male, the megagamete the female conjugating body (see 

 P- 173)- 



