276 VORTICELLA CHAP. 



the cyst bursts, and a number of small bodies or spores 

 (H 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, and they 

 are sometimes found to multiply 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 6 ), this increases in length, the basal circlet 

 of cilia is lost, and a ciliated peristome and disc are 

 formed at the free end (H 7 ). In this way the ordinary 

 form is assumed by a process of progressive differentia- 

 tion or development (p. 9) ; and, moreover, the free- 

 swimming young (H 3 ), 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, how- 

 ever, 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 down- 

 wards 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. 



