FREE-SWIMMING ZOOIDS 129 



direction the direction of length of the fibre so that its 

 inevitable result is to shorten the fibre and consequently to 

 bring its two ends nearer together. This is the essential 

 characteristic of a muscular contraction, and the axial fibre 

 in the stalk of Vorticella is therefore to be looked upon as 

 the first instance of a clearly differentiated muscle which has 

 come under our notice. 



There are some interesting features in the reproduction of 

 Vorticella. It multiplies by binary fission, dividing through 

 the long axis of the body (Fig. 26, E 1 , E' 2 ). Hence it is 

 generally said that fission is longitudinal, not transverse, as 

 in Paramcecium. But on the theory (p. 107) 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 1 ), and gradually 

 deepening until there are produced two complete and full- 

 sized 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 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 



K 



