100 SURVEY OF THE REPRODUCTIVE PROCESS 



appearance, however, of the embryonic structures, gemma- 

 tion comes into play, manifesting itself primarily and most 

 extensively in the multiplication of the segments of the 

 body. The species with external branchia quit the egg as 

 short ciliated larva-like Infusoria, and only acquire the ver- 

 miform character by the successive gemmation of segments 

 from behind, much as the joints forming the " body" of the 

 Tapeworm are developed from the back of the Tsenia-head. 

 Only there is this difference, that, with the exception of the 

 caudal appendage, the segments are budded oif from each 

 other, not all from the head, so that the penultimate is 

 always the most recent, while in the Cestoids the posterior 

 segments are the oldest. 



In both cases it is only in these derivative segments that 

 generative organs are developed, and the parallel is carried 

 still farther, when, as in Syllis, they are thrown off as sexual 

 zooids, the main difference being that in the Annelida the 

 offsets have more the character of distinct animals. The 

 joints are not only thrown off in sets, like the catenated 

 zooids of the Salpce, but such an organic unity is established 

 among the connected segments as to give each set quite the 

 character of a complete animal. In these cases one or more 

 of the later segments become secondary foci of a budding 

 process, which proceeds exactly in the same way as in the 

 young annelidan first formed from the ovum ; that is, the 

 ring which is to develope the new zooid sub-divides into 

 two parts, which acquire the organization respectively of 

 head and tail segments, but it presents at first no farther 

 division, the other joints being gradually formed afterwards, 

 between the cephalic and caudal extremities, and always in 

 succession from the posterior of the segments previously 

 produced. Phenomena of this kind were first observed by 



as its own mouth. From this body the annelidan is probably developed, 

 after the plan which prevails generally in the class. Huxley in Medical 

 Times, XIII., 281. 



