ALTERNATION OF GENERATIONS. 139 



animal formed solely to serve as a reproductive ma- 

 chine."* 



The zooids, it is to be observed, are not derived from 

 each other, as the rings themselves are, but are the results 

 of a separate budding process in certain of tlie posterior 

 segments ; and the conversion of segments into zooids takes 

 place in the reverse order of their original development — 

 that is, from behind forwards. Though the liindmost joint 

 is the last formed, its derivative Annelidan is the first ma- 

 tured.-|- 



The sequence of alternation here is, therefore, parallel 

 to that occurring in the Polypifera, only there is less dif- 

 ference between the sexual zooids and the parent stock — 

 the annelidan zooids are neither so rudimentary as some of 

 the medusoids, nor do they ever rise so much above the 

 level of the stock as the large hood-eyed Medusaj. Both 

 the parent animal and its offsets have the obvious annelidan 

 structure, yet there are certain minor distinctions between 

 them — such as mio-ht even indicate a o-eneric difference, and 

 it has been ascertained that the young produced from the 

 ova resemble, not the immediate parents, but the primary 

 Annelida, in their general conformation, as well as in their 

 want of sex and their tendency to caudal gemmation. J 



Among the Annehda, therefore, we seem warranted in 

 saying that gemmation may occur in all the three stages 

 before specified,§ though it is only as an occasional occur- 



* Rambles of a Naturalist, I., 217. 



f Hence the facetious remark of Mr. Lewis is hardly in point, that the 

 family inheritance of the parental tail, in passing to the progeny, re- 

 verses the law of primogeniture, and like the stock of baby linen, descends 

 always to the youngest. Sea-side Studies, p. 62. 



X Milne Edwards, Ann. des Sciences Nat., 3d Ser., Zool., torn iii., p. 170. 



§ Terehellay in which the gemmation of segments from the infusorial 

 germ, and from each other, has been most distinctly observed, presents 

 also, according to Mr. Lewis, the formation and detachment of caudal 

 zooids. Sea-side Studies, 62. 



