350 ANIMAL BIOLOGY. [Part II. 



One mode of multiplication adopted by vorticella is that of 

 fission (Fig. 102, C.). The animal assumes the pear-shaped con- 

 dition of contraction, and becomes somewhat swollen. A notch 

 then makes its appearance in such a way as to divide the vesti- 

 bule into two halves. The notch becomes a cleft, and the 

 cleavage is continued through the body in such a way as to 

 divide the contractile vesicle and the band-like endoplast. When 

 the cleavage is complete, there result two heads to the one stalk. 

 But the fate of the two is not the same. One remains attached 

 to the pedicle and expands into a complete vorticella. The 

 other remains pear-shaped, and develops round the posterior 

 region of the body a girdle of powerful vibratile cilia, by the 

 lashing of which the zooid tears itself away' from the parent 

 stem and swims off through the water (Fig. 102, D.). After a 

 short active existence it settles down in a convenient spot, 

 adhering by its posterior extremity. The hinder girdle of cilia 

 is lost or absorbed, a stalk is rapidly developed, the peristome 

 expands, the adoral wreath of cilia exhibit their orderly activity, 

 and the organism is a perfect vorticella. 



In the case just described, the two zooids which result from 

 fission are of equal size. But in some cases the fission is very 

 unequal, so that a minute microzooid results. This swims off by 

 means of its posterior girdle of cilia, but does not merely settle 

 down and grow up into a vorticella. Its mission is different. 

 It attaches itself to the base of the bell of another vorticella, and 

 becomes completely absorbed into its larger mass. Such a pro- 

 cess is called conjugation. Perhaps we may see in this process 

 a foreshadowing of the union of spermatozoon and ovum in 

 the metazoa. The result of the process would seem to be a 

 rejuvenescence or access of vital power ; for the normal mode of 

 multiplication by binary division would seem to go on more 

 rapidly after conjugation. The endoplasts of the two conju- 

 gating organisms seem to break up and disintegrate, a new 

 endoplast being shortly afterwards reconstructed. 



It is not improbable that, in some cases, conjugation is a pre- 

 paratory process to encystment. But it would seem that encyst- 

 inent may take place without any such preparatory process. 



