Phytogeny of the Organization of the Gestoda. 419 



tation ; following these we have the Bothriocephaliche, with 

 short but sharply defined joints, which, however, are not yet 

 set free as separate units, but are liberated from the body of 

 the Cestode in larger sections after the sexual organs have 

 arrived at maturity. A higher stage of individualization is 

 reached in the Tamiadse, where the proglottids are set free 

 singly ; and, lastly, the highest stage of all is attained in the 

 case of many Phyllobothridas, the joints of which undergo a 

 further development after separation, with considerable increase 

 in size, and are capable of independent existence for some 

 time {Echinohothriitm) . 



In spite of the similarity which exists between the alterna- 

 tion of generations in the Cestodes and that in the Acalephae 

 (Scyphomedusas), a similarity so complete that the same term 

 " strobila" is applied to the segmented stage in both cases, 

 the origin of it in each case requires a very different explana- 

 tion. The alternation of generations in the Scyphomedusas, 

 which are asexually produced as sections of a polype which 

 segments and forms the strobila, appears, as compared with 

 the simple direct development of certain Medusaa {Pelagia 

 noctiluca), to be a primeval developmental process of palin- 

 genetic importance. Accordingly the ephyra, which is set 

 free by fission at the distal end of the strobila, represents when 

 contrasted with the young polype the morphologically higher 

 and more perfectly organized form. The exceptional case of 

 direct development, which is found in Pelagia noctiluca owing 

 to omission of the strobila-stage, is to be regarded, on the 

 other hand, as an entirely secondary condition, derived from 

 the alternation of generations by a shortening of the develop- 

 mental process. 



In contradistinction to the Medusa set free at the distal 

 end of the Acalephse-strobila, the proglottis liberated from the 

 Cestode-strobila represents, when compared with the ancestral 

 Trematode, a lower form, simplified and to a certain extent 

 retrograded by the disappearance of the organs of adhesion 

 and of the alimentary canal, though in point of fact the 

 reduction of the organs was a condition of its individualiza- 

 tion. While in the former instance the alternation of gene- 

 rations is the original and primary process, and the completion 

 of the metamorphosis in the same individual the later one, 

 secondarily produced by shortening and simplifying the deve- 

 lopment, in the Cestodes exactly the opposite is the case, and 

 the alternation of generations is the later form of develop- 

 ment, secondarily derived from the metamorphosis which was 

 formerly undergone by one and the same individual, in con- 

 nexion with the simplification of the organization and the 



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