ALTERNATION OF GENERATIONS. 137 



both in the protomorphic and gamomorphic stages ; as also, 

 in a more latent form, the Polyzoa, which will come under 

 consideration again, as illustrating the connection between 

 alternation of generations and certain phenomena occurring 

 in the higher animals. 



8. Some Annelida may be said even to present pheno- 

 mena akin to those of alternation in all the three. The 

 earlier alternations are most readily to be traced in the 

 branchiated species. In these the primary infusorial form 

 in which the young quit the egg may be compared to a 

 protomorphic zooid, such as the " six-hooked embryo" of 

 the Cestoidea. As the latter buds off a tsenia-head, so 

 this infusorian gives origin, on one side, to the cephalic, 

 and, on the other, to the caudal and penultimate segments 

 of the Annelidan conjointly representing the commence- 

 ment of the typical form after which the ciliated part 

 gradually wastes away. There now follow a series of suc- 

 cessive acts of gemmation, the penultimate segment budding 

 off a new joint on its caudal aspect, which in turn repeats 

 the process, and so on, till there is formed eventually a 

 long train of such segments, each of which (except the tail 

 piece), is derived by gemmation from the one immediately 

 in advance of it. Hence, all the links of this later gem- 

 mation are as distinctly referable to the orthomorphic phase 

 as are the successive gemmations of Aphides, from which 

 they differ only in continuing in organic union with each 

 other, instead of assuming the position of free zooids. 



This correspondence is much closer than that which 

 subsists between the Annelidan and the Cestoid worm. 

 Much as these resemble each other in the composite cha- 

 racter of the long vermiform body, made up of a series of 

 derivative and semi-independent gemmse, there are two 

 important points of difference between them. 



1 . The segments of the Cestoid worm, though of different 

 ages and degrees of maturity, are all originally budded off 



