METAMORPHOSIS 131 



unite to form a zygote, a motionless body provided with a 

 cell-wall, the protoplasm of which divides into spores : in 

 Paramcecium (p. 113) no zygote is formed, conjugation being 

 a mere temporary union : in Vorticella the zygote is an 

 actively moving and feeding body, indistinguishable from an 

 ordinary individual of the species. 



Vorticella sometimes encysts itself (Fig. 26, H 1 ), and the 

 nucleus of the encysted cell has been observed to break up 

 into a number of separate masses, each doubtless surrounded 

 by a layer of protoplasm. After a time the cyst bursts, and 

 a number of small bodies or spores (n 2 ) emerge from it, each 

 containing one of the products of division of the nucleus. 

 These acquire a circlet of cilia (H 3 ), by means of which they 

 swim freely, sometimes multiplying by simple fission (H 4 ). 

 Finally, they settle down (H 5 ) by the end at which the cilia 

 are situated, the attached end begins to elongate into a stalk 

 (H), this increases in length, the basal circlet of cilia is lost, 

 and a ciliated peristome and disc are formed at the free end 

 (H T ), and in this way the ordinary form is assumed by a 

 process of development recalling that we found to occur in 

 Heteromita (p. 42), but with an important difference : the 

 free-swimming young of Vorticella (H S ), to which the spores 

 formed by division of the encysted protoplasm give rise, 

 differ strikingly in form and habits from the adult. This is 

 expressed by saying that development is in this case accom- 

 panied by a metamorphosis, this word literally meaning simply 

 a change, being always used in biology to express a striking 

 and fundamental difference in form and habit between the 

 young and the adult ; as, for instance, between the tadpole 

 and the frog, or between the caterpillar and the butterfly. It 

 is obvious that in the present instance metamorphosis is 

 another means of ensuring dispersal. 



In Vorticella, as we have seen, fission results not in the 



K 2 



