278 VdRTICELLA chap 



in Paramoecium. But on the theory (p. 275) that the peris 

 tome and disc are dorsal and the attached end ventral, 

 fission is really transverse in this case also. 



It will be seen from the figures that the process takes place 

 by a cleft appearing at the distal end (e') and gradually 

 deepening until there are produced two complete and full- 

 sized individuals upon a single stalk (e^). This state of 

 things does not last long : one of the two daughter-cells takes 

 on a nearly cylindrical form, keeps its disc and peristome 

 retracted, and acquires a new circlet of cilia near its proximal 

 end (e') : it then detaches itself from the stalk, which it 

 leaves in the sole possession of its sister-cell, and swims about 

 freely for a time in the direction indicated by the arrow. 

 Sooner or later it settles down, becomes attached by its 

 proximal end, loses its basal circlet of cilia, and develops a 

 stalk, which ultimately attains the normal length. 



The object of this arrangement is obvious. If, when a 

 Vorticella divided, the plane of fission extended down the 

 stalk until two ordinary fixed forms were produced side by 

 side, the constant repetition of the process would so increase 

 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 (p. 272); 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 A^orticella divides either into two unequal 

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

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



