VORTICELLA I CONJUGATION 59 



eight micronuclei. In each conjugant all but one of these perish and 

 the survivor divides into two, which correspond to the male and 

 female pronuclei of Paramecium. This division takes place while 

 the two micronuclei are lying in the region where the endoplasm 

 of the conjugants became continuous. One half of each micro- 

 nucleus passes into the larger conjugant, where the two fuse as 

 male and female pronuclei. The other half of each passes into the 

 smaller conjugant, but these halves, instead of fusing, degenerate 

 and disappear. The endoplasm of the small exconjugant is now 

 drawn into the larger, the ectopasm shrivelling up and falling 

 off. It will be seen that the conjugation of Vorticella takes place 

 in the same way as that of Paramecium, but that one of the two 

 exconjugants perishes and is partly absorbed by the other. ^ 



OTHER STALKED CILIATES 



Carchesium is a small freshwater animal whose body consists 

 of a number of members, each of which has the structure of a 

 whole Vorticella. It arises from a Vorticella-like body, by divisions 

 like those which take place in the ordinary reproduction of 

 Vorticella, save that the division passes some way down the stem 

 and then stops, leaving the bells joined by their stalks. Thus 

 the body is increased by the addition of new members which 

 repeat the structure of the old. The whole body of a Carchesium is 

 said to be a colony, and its members are zooids. Reproduction is 

 brought about by the complete fission from the body of certain 

 zooids, which thus become asexually produced young (buds). 

 Each of these swims off, settles down, and forms by growth and 

 nuclear division a new colonial individual. Conjugation like that 

 of Vorticella also takes place. Each bell of Carchesium has its own 

 myoneme. and contracts independently of its neighbours ; 

 Zoothaminium is much like Carchesium, but there is one con- 

 tinuous branched myoneme, and the whole colony contracts 

 together. Epistylis is colonial but non-contractile. Many species of 

 Epistylis and some of Carchesium are epizootic on freshwater 

 Crustacea and other animals. 



^ The student should beware of comparing the smaller conjugant of Vorticella 

 with a spermatozoon and the larger with an ovum. Ova and spermatozoa are 

 gametes of unlike kinds. The conjugants of Vorticella are unUke, hermaphrodite 

 parents, each of which forms two unlike gametes. 



