104 MORPHOLOGICAL DEVELOPMENT. 



arising by repeated longitudinal budding, which after reach- 

 ing certain lengths undergo spontaneous fission: in some 

 cases doing this so as to form two or more similar strings 

 of segments constituting independent individuals; and in 

 other cases doing it so that the segments spontaneously 

 separated are but a small part of the string. Thus a Syllis, 

 Fig. 168, after reaching a certain length, begins to trans- 

 form itself into two individuals : one of the posterior segments 

 develops into an imperfect head, and simultaneously narrows 

 its connexion with the preceding segments, from which it 

 eventually separates. Still more remarkable is the extent to 

 which this process is carried in certain kindred types; which 

 exhibit to us several individuals thus being simultaneously 

 formed out of groups of segments. Fig. 169, copied (omit- 

 ting the appendages) from one contained in a memoir 

 by M. Milne-Edwards, represents six worms of different 

 ages in course of development: the terminal one being the 

 eldest, the one having the greatest number of segments, 

 and the one that will first detach itself; and the success- 

 ively anterior ones, with their successively smaller numbers 

 of segments, being successively less advanced towards fitness 

 for separation and independence. Here among groups of 

 segments we see repeated what in the previous cases occurs 

 with single segments. And then in other annelids we find that 

 the string of segments arising by gemmation from a single 

 germ becomes a permanently united whole: the tendency to 

 any more complete fission than that which marks out the seg- 

 ments, being lost; or, in other words, the integration having 

 become relatively complete. Leaving out of sight the 



question of alliance among the types above grouped together, 

 that which it here concerns us to notice is, that longitudinal 

 gemmation does go on; that it is displayed in that primitive 

 form in which the gemma? separate as soon as produced ; that 

 we have types in which such gemma? hang together in 

 groups of four, or in groups of eight and ten, from which 

 however the gemmae successively separate as individuals; 



