274 VORTICELLA CHAP. 



duction of Vorticella.. It multiplies by binary fission, 

 dividing through the long axis of the body (E 1 , E 2 ). 

 Hence it is generally said that fission is longitudinal 

 not transverse, as in Paramoecium. But on the theory 

 (p. 271) that the peristome 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 1 ) and 

 gradually deepening until there are produced two 

 complete and full-size individuals upon a single stalk 

 (E 2 ). 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 3 ) : 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. 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 : contrivances having this object in 

 view are a very general characteristic of fixed organisms. 



Conjugation occasionally takes place, and presents 



