386 THE DEVELOPMENT OF CESTODES. 



sexual segment. These three forms represent as many generations, and 

 all three form together a life-cycle. We have already given reasons 

 for regarding the two latter forms as individuals, and the claims of 

 the six-hooked embryo will be doubted by no one who considers its 

 history without bias. The sexual animal results by asexual reproduc- 

 tion from the tape-worm head, and remains for a period in union with 

 it, forming the worm-chain or Strobila. In the same way the tape- 

 worm head has its origin in the embryo. The manifold metamorphoses 

 undergone by the latter do not in any way affect its individuality. 

 The caudal bladder of the Gysticercus is, like the homologous Echino- 

 coccMs-bladder, a morphologically independent structure ; it stands in 

 the same relation to the tape-worm head as the latter to the sexual 

 animal. Like the jointed tape-worm, the typical bladder-worm is 

 morphologically not a single individual, but a colony, with joints at 

 different stages of development. 



Of the three different generations which are distinguishable in the 

 Cestode life-cycle, only one — the Proglottis — is sexual. Both the others 

 are preparatory, and have only the power of asexual reproduction. 



The life-history of the Cestodes seems, therefore, to be an alterna- 

 tion, with two " nurse " forms — one a " nurse " in the proper sense of 

 of the term, the Scolex, the other a " grand-nurse " (Grossamme), the 

 six-hooked embryo. 



We should, however, be doing violence to nature if we tried to 

 apply this formula rigidly in all instances. For, besides the' forms 

 in which we can observe the three successive generations in sharp and 

 distinct contrast, there are others in which these developmental stages 

 have so little independence, and follow one another so gradually, that 

 it is impossible for us any longer to credit them with distinct indi- 

 viduality. The structural variations, elsewhere spread over diiferent 

 generations, appear in these cases merely as developmental phases 

 of the same individual : the alternation of generations is replaced 

 by a metamorphosis. 



We have in the course of our survey noted many of these cases, 

 — some where it was impossible to distinguish nurse and grand- 

 nurse as individualised structures, and others where nurse and 

 sexual animal were bound up in one. The Scolex-loxms may suffice 

 for illustration of the former case, and Archigeies of the latter. 

 Perhaps we should also regard the LigulidiB as forms in which the 

 three otherwise separate generations are condensed into one, so that 

 we cannot speak of an alteration of generations. 



In the light of the many surprising results of the study of animal 

 development, these facts do not seem specially unique. We know how 

 in the lower anima&;^te^ili6^]lt}ftjirai]s'6/l®s often only a restricted 



