420 Prof. C. Claus on the Morphology and 



more favourable conditions of nourishment and growth owing 

 to intestinal parasitism. It follows that we have to define 

 the development of the Acalephae as alternation of genera- 

 tions *, which in certain cases can be simplified by shortening 

 into metamorphosis, while we must interpret the development 

 of the Cestodes, on the other hand, as metamorphosis, which, 

 owing to individualization of certain products of growth, can 

 give rise to variously complicated forms of alternation of gene- 

 rations. 



However, we have not yet taken into account the first most 

 variable series of manifold and complicated developmental 

 stages, by which the embryo produced from the fertilized 

 ovum is transformed into the scolex. But it is precisely this 

 portion of the ontogeny which is of peculiar importance for 

 our problem, not only because by the appearance of a stage 

 capable of asexual reproduction, and therefore distinguished 

 as the primary nurse-form, the complication of the develop- 

 mental processes interpreted as an alternation of generations 

 is increased and the justice of such an interpretation thereby 

 strengthened, but also because it is just this section of the 

 development which is to be brought into direct comparison 

 with that of the Trematodes. And if we are right in tracing 

 the phylogeny of the Cestodes from the Trematodes, the 

 development of the latter, which for a long time was itself 

 considered as alternation of generations, must be repeated, 

 though in a modified form, in their descendants. 



Every one is aware that the digenetic Trematodes — and 

 these alone, and not the monogenetic forms, can be considered 

 in the comparison — as well as the Cestodes pass their imma- 

 ture stages in other hosts than those of the sexual animals, 

 whereby a change of hosts becomes a necessity. In the case 

 of the former it is usually a mollusk or other Invertebrate in 

 whose body the intermediate generations develop themselves 

 as the so-called " sporocysts " or " redia3," according as they 

 do or do not possess a mouth and alimentary canal, together 

 with their progeny, the " Cercarige " or Distoma-larvaj. In 

 the case of the Cestoda, too, the part of first host is sometimes 

 played by an Invertebrate, but generally it is a Vertebrate in 

 which the young forms are to be found as " Cysticercoids " or 



* It is owing to the astonishing similarity existing between the pheno- 

 mena of growth and fission in the Acalepha3- and Cestode-strobilas, and 

 between the formation of ephyrse and proglottids, that the phylogenetic 

 contrast has been overlooked and the morphological value of the sexual 

 generations in both cases identified, and that the mistake has then been 

 made (Gotte) of not admitting the development of the Acalephae to be 

 an instance of alternation of generations, but interpreting it as a meta- 

 morphosis. 



