THE PROTOZOA 17 



After the conjugation, as has been already said, multiplication 

 by simple fission goes on with increased activity. 



The differentiation seen in Vorticella between the large 

 passive macrogamete and the small active microgamete, is one 

 that is retained in the female and male gametes of the more 

 highly organised multicellular forms, where, however, the 

 gametes do not consist of entire individuals, as is frequently 

 the case in unicellular forms, biit only of special cells dif- 

 ferentiated in the body of each individual for the purpose of 

 such fusion. The fusion, moreover, results in the formation 

 of a now separate organism, arising from each pair of fused 

 gametes (see Hydra, page 30). 



Occasionally Voiticella may surround itself with 

 ^aee^ a protective " cyst," and remain dormant within it 

 for some time, and such encystment may be followed, 

 though only rarely, by the breaking up of the protoplasm 

 within the cyst into a number of small ovoid bodies known 

 as spwes.^ ' Each of these spores develops a single circlet of 

 cilia, and, breaking free, swims away, sometimes multiplying 

 by fission, but eventually settling down — as in the case of 

 the free bells formed by simple fission — growing a stalk, and 

 gradually becoming a normal individual with stalk, bell, disc, 

 gullet, and all the other characters of an ordinary Vorticella. 

 This is a simple case of development, the term used to signify 

 the gradual growth of the adult form from a germ unlike 

 itself. 



In some closely allied forms — e.g. tlie common 



Forms fresh -water Carchesium spedabile, in which the 

 structure of each individual is very like that of 

 Vorticella — when fission takes place, it involves the stalk also, 

 to some extent, and, further, the daughter forms or zooids all 

 remain attached to the paren t ; in this way a branching 

 colony of forms results ; each individual, however, being 

 quite independent of the others, so far as all its vital pro- 

 cesses are concerned (Fig. 8). In Carchesium, all the indi- 

 vidual zooids are similar in size and shape, but in some other 

 such compound organisms or colonies, there is a distinct 

 division of labour amongst the individuals, some being con- 

 cerned only with the nutrition of the colony, whilst others 

 are mouthless, have a basal circlet of cilia, and are concerned 



1 Allmanu, Q.J. M.S., New Series, vol. xii., 1872, p. 393. 



C 



