276 VORTICELLA CHAP. 



swim freely, and they are sometimes found to multiply by 

 simple fission (n 4 ). Finally, they settle down (i* 5 ) by the 

 end at which the cilia are situated, the attached end begins 

 to elongate into a stalk (H G ), 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 ). In this way the 

 ordinary form is assumed by a process of progressive 

 differentiation or development (p. 9), and, moreover, the 

 free-swimming young (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 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, 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 

 Carchesium and Epistylis. Each of these forms consists of a 



