CILIATA. 



41 



The most usual method of reproduction is by fission. When the 

 forms reproduced remain connected together, a colony is formed, e.g., 

 the colonies of Epistylis and Carchesium. Fission usually takes place 

 by a transverse division (at right angles to the long axis), and, the 

 products may be equal or unequal. In cases of inequality we have 

 transitions to budding, which is really only a modified fission. Less 

 frequently ( Vwiicella) the fission takes place through the long axis 

 (Fig. 36), and still more rarely in a diagonal 

 direction. The onset of fission does not 

 appear to depend upon the size of the indi- 

 vidual, for it may take 

 place in large or in small 

 specimens. The nuclei 

 always participate in the 

 division, but do not 

 always lead the way ; 

 for in many cases un- 

 doubted new formations 

 appear in the protoplasm 

 (rudiments of new cili- 

 ary structures, of a 

 mouth and contractile 

 vacuole), before any 

 changes are observed in 

 the macro- and micro- 

 nuclei. As to the Other FIG - ^5. - Pa^maxium 

 aurelia in division, 



organs they do not par- after R. Hertwig. N 



macronucleus ; n micro- 

 nucleus ; o mouth of 



FIG. 34. Stylonychia mytilus in 

 C con- 



ticipate in the division 



division, from Stein. 



tractile vacuole ; N macro- -\yhole 



nucleus ; n micronuclei. 



the anterior portion ; 

 N', n', o 1 the same of 

 the hinder portion. 



(unless they extend the 

 length of the 

 body), but one of the 

 fission products develops them afresh, generally before the fission 

 is completed. In the Hypotriclia the ciliary structures of both the 

 products of fission appear to be new formations. 



In Opalina the nucleus divides many times, so that a multinucleate 

 condition is produced (Fig. 33). The gradual division of the whole 

 animal by a series of binary fissions into a number of small pieces 

 which encyst, takes place subsequently. From the cyst a small 

 uninucleate form eventually emerges. 



Reproduction is often preceded by envtjutim'ut, which appears to be 

 of great importance for the preservation of the body from desiccation. 



