138 THE NATURE AND VARIETIES OP 



from one point the back part of the Tsenia-head and 

 therefore, except in so far as they have multiplied after- 

 wards by sub-division, may be said to constitute but a 

 single generation, being all the progeny of one parent stock, 

 while the segments of the Annelidan are a succession of 

 generations, each being derived from that immediately in 

 front of it ; so that the hinder ones are the youngest, not, 

 as in the Tsenia, those nearest the head. 



2. The segments of the Cestoid worm are clearly gamo- 

 morphic or sexual gemmse, for as soon as they show any 

 distinct organization at all, it tends to the development of 

 the reproductive elements. Those again of the Annelidan 

 are, as has been observed, orthomorphic, tending to the 

 completion of the typical body, which attains some con- 

 siderable elongation before it enters on the sexual stage. 

 As in general it is only the later segments that develope 

 reproductive organs, these indeed may with some reason be 

 considered as constituting a new series of gemmae, really 

 corresponding to the gamomorphic segments of the Tape- 

 worm. In Syllis, Myrianida, and some other species, it is 

 obviously so, for the later segments become, as has been 

 already noticed, so many new centres of a budding process, 

 resulting in the formation and detachment of distinct sexual 

 zooids. In this case the primary Annelidan has no sexual 

 organs, but the caudal zooids become gorged with ova, even 

 before actual separation takes place. In the same way 

 other individuals of the species, equally destitute themselves 

 of sexual parts, throw off from their posterior extremity 

 secondary Annelida, with voluminous spermatic organs. 

 When impregnation has been effected, and the ova have at- 

 tained a certain degree of development, the female zooid 

 becomes completely detached from the parent stock. Its 

 separate existence, however, is of limited duration, for it is 

 ruptured by the growth of the eggs, which are thus dis- 

 persed. As M. Quatrefages expresses it, the zooid " is an 



