20 HANDBOOK OF INVKKTKHRATE ZOOLOGY. 



I. The Multiplication by Fission. 



a. Occasionally a Vorticella becomes permanently re- 

 tracted, and the body becomes lengthened laterally ; the 

 peristome gradually disappears ; the nucleus becomes 

 more conspicuous ; the food vacuoles and granules gradu- 

 ally disappear; the sarcode becomes transparent; and, 

 after a time, the nucleus assumes a position at right .-in- 

 gles to the stem, and the body shows traces of a vertical 

 division into two, as shown in Fig. 7. 



b. The nucleus soon divides into two portions, which 

 separate from each other to become the nuclei of the two 

 new animals. (Fig. 8). 



c. The constriction next becomes more marked, and at 

 or near each end of the long axis of the compound body 

 a curved groove makes its appearance. This groove soon 

 shows traces of ciliary action, and becomes converted into 

 the peristome of one of the new animals. 



d. The animals then become completely separated, as 

 shown in Fig. 9. They assume the vase-like shape. 

 The peristomes and discs become fully developed, and two 

 perfectly-formed Vorticellae are now mounted upon a sin- 

 gle stem. 



e. The stem gradually becomes forked. 



f. Each of these animals may soon repeat the same 

 process of division, thus building up a community by re- 

 peated fission. 



II. The Formation of the Free Form. 



a. Sometimes, after the completion of the division, one 

 of the new animals is smaller than the other, and is situ- 

 ated nearly at right angles to the common stem. 



b. This soon develops a crown of cilia around the fixed 

 end of the body, as shown in Fig. 10. 



c. It then detaches itself from the stem by violent 

 movements, and swims away by means of its cilia. 



