28o VORTICELLA chap. 



These acquire a circlet of cilia (h^), by means of which they 

 swim freely, and they are sometimes found to multiply by 

 simple fission (h*). Finally, they settle down (h^) 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''). In this way the 

 ordinary form is assumed by a process of development 

 (p. 273), and, moreover, the free-swimming young (h^), to 

 which the spores formed by division of the encysted proto- 

 plasm give rise, differ strikingly in form and habits from the 

 adult. This is expressed by saying that development is in 

 this case accompanied by a metamorphosis, this word literally 

 meaning simply a change, being always used in zoology 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 (p. 9) 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 

 production of equal and similar daughter-cells, but of one 

 stalked and one free-swimming form. It is however quite 

 possible to conceive of a Vorticella-like organism in which 

 the parent cell divides into two equal and similar products, 

 each retaining its connection with the stalk. If this process 

 were repeated again and again, and if, further, the plane of 

 fission were extended downwards so as to include the distal 

 end of the stalk, the result would be a branched, tree-like 

 stem with a Vorticella-like body at the end of every 

 branch. 



As a matter of fact, this process takes place not in Vorti- 

 cella itself, but in some nearly allied infusors, such as 



