CESTOIDEA. 9 



seen that they constitute three distinct individua- 

 lities, of which one, at least, is derived from another 

 by gemmation. 



The head, or scolex, evidently possesses a special 

 individuality. It is distinguished from each of the 

 rings by its form, by its suckers, by the constant 

 absence of sexual organs, and often by the presence 

 of hooks ; and, if in certain species, it seems to 

 belong to the series of rings because it is not plainly 

 separated from them, the separation is well-marked 

 in other species ; besides which, the head of certain 

 cestoid worms has been seen detached, and has even 

 been described as a distinct animal under the name 

 of scolex. 



- The rings, or proglottides, also possess a peculiar 

 individuahty, which is clearly shown in a considerable 

 number of species, as, after they have remained for 



twelve successive phases of generation, but two only, one sexual 

 and the other non-sexual ; the hydatids produced successively 

 from one another do not each represent a new phase of genera- 

 tion, but it is the echinococcus which represents this new phase ; 

 just as in plants, the succession of buds only represents the same 

 phase of generation. 



Steenstrup, the author of the theory of Alternate Generation, 

 calls the non-sexual individual, which gives birth to the sexual 

 one, the nurse ; and lie designates as grand-nurse, the non-sexual 

 one, which, when there are two non-sexual phases of development, 

 gives birth to the nurse. Van Beneden calls the nurse scolex, 

 and the grand-nurse proscolex. 



Amongst the entozoa, the cestoid and trematode worms are 

 generally propagated by alternate generation ; but the different 

 phases of their generation are accomplished in different situa- 

 tions. The animal cannot pass through the stages of larval 

 existence in the organ in which it becomes adult, and there is 

 consequently a necessity for migration into new organs and new 

 animals, this migration corresponding to each new phase of its 

 evolution. 



