Phylogeny of the Organization of the Cestoda. 421 



" Cysticerci " — the latter usually encysting in parenchyma- 

 tous organs. There can therefore be no question but that we 

 must trace back Cysticercus as well as Cysticercoid either to 

 the Cercaria-producing redia or sporocyst, or else to the 

 Cercariaj themselves, provided that we admit the origin of the 

 Cestodes from the Trematodes. 



The choice between the two alternatives appears at first 

 sight to be by no means an easy one ; but, taking into con- 

 sideration the analogy of the proliferation of the Cysticercus, 

 it would seem to rest with the redia or sporocyst as the 

 equivalent of the latter. However, a closer comparison shows 

 us that in this proliferation we have to deal merely with 

 analogous and not with homologous processes, since the germs 

 produced by the sporocysts and redia}, which were formerly 

 regarded as spores, or even as internal buds, cannot be placed 

 on the same level as the buds on the wall of the Cysticercus- 

 vesicle, but must be considered as formations of quite a diffe- 

 rent kind. It is true that for a long time (that is to say as 

 long as the theory that the Distoma-development was a case 

 of alternation of generations remained undisputed) these forma- 

 tions were regarded as products of an asexual reproduction, 

 that is as spores or buds, until the discovery of pedogenesis 

 in Diptera-larvge, and the precocious separation of the first 

 sexual cells, which sometimes even takes place during the 

 segmentation of the oosperm *, led to an entirely different 

 view — a view which, supported by the consideration that 

 spore-formation in Metazoa is a priori highly improbable, 

 has now come to be the one which is generally accepted. In 

 accordance with this view the so-called germ-cells in the 

 sporocysts and redia; are considered to be ovarian cells 

 separated early and developing in the body of the larva ; that 

 is to say, they are held to be ova developing parthenogeneti- 

 cally t 3 and the development of a Distoma is no longer 

 explained as a case of alternation of generations, but as a form 

 of heterogamy. The sporocysts and rediae would then be 

 explained simply as larval forms which have undergone a 

 retrogressive metamorphosis or else have been checked in their 

 development for the purpose of aiding the rapid and extensive 

 reproduction of individuals j they would correspond to larval 



* Compare C. Grobben, " Die Entwicklungsgeschichte der Moina 

 reciirostris" Arbeiten aus dern Zoologischen Institute in Wien und der 

 Zoologischen Station in Triest : Vienna, 1879, Bd. ii. 



t This interpretation, first given by Grobben (loc. cit.), has since been 

 repeated by other investigators also in a precisely similar way, and may 

 be said to have met with pretty general acceptance at the present day. Cf. 

 also H. Schauinsland's ' Beitrag zur Kenntniss der Embryonalentwicke- 

 lung der Trematoden,' Jena, 1883. 



