428 Prof. C. Claus on the Morphology and 



closely-allied species of animals are found as the intermediate 

 hosts of the same species of worm, and that the same thing 

 also recurs in the case of the hosts of the sexually mature 

 Helminthes. 



Now if as early as in the case of the Trematodes, which are 

 phylogenetically to be derived from Dendrocoele Turbellaria, 

 the intermediate hosts were not the original hosts of the 

 sexual worms, much less can this have been the fact with 

 regard to the Cestodes which have been developed from them ; 

 neither Cysticerci nor Cysticercoids in the bodies of their 

 victims will ever have represented the terminal stages with 

 digenetic reproduction in the life-history of these Helminthes. 

 Like the larva? of the Trematodes the young stages also of 

 the oldest Cestodes, living in the intestines of fish and other 

 aquatic Vertebrata, penetrated the bodies of Invertebrates, 

 and there transformed themselves, instead of Redias and Cer- 

 cariee, into Cysticercoids. 



It was only later on, with the appearance of birds and 

 mammals, that the Tamiadae came into existence, the larval 

 stages of which remained only to a limited extent in Inverte- 

 brates, but in the majority of cases migrated into the bodies 

 of Vertebrata, in which from Cysticercoids they developed 

 into Cysticerci. The reader will be reminded by these obser- 

 vations of the doctrine of von Siebold, who regarded the 

 Cysticerci as tapeworms gone astray into the wrong animals, 

 becoming in a strange abode dropsical and degenerate, whereby 

 he for a long time denied the value of the Cysticerci as 

 normal larval stages of Cestodes. As a matter of fact we 

 might just as well speak of going astray in the case of phylo- 

 genetic development as in that of free-living animals, certain 

 individuals of which are cast away beyond the limits of 

 distribution of the species into domains far distant and sepa- 

 rated by mighty barriers, and there, as a result of the entirely 

 altered conditions of subsistence, give rise to the development 

 of new species and groups. And since in physiology no hard- 

 and-fast line is to be drawn between the normal and the patho- 

 logical, and only so far in theory, as the latter processes bring 

 with them disturbing results detrimental to the life of the indi- 

 vidual, we should even hold the conception of the dropsical 

 degeneration to be justified *, though certainly in a sense very 

 different from that of Siebold's doctrine, which was entirely 

 opposed to the idea of transmutation, and, as compared with 

 the results of Kuchenmeister and R. Leuckart's investigations, 

 merely defended an error. It is therefore a serious and 



* C. Claus, 'Gnmdziige der Zoologie,* 4th edition, part ii. 1879, p. 389. 



