Phylogeny of the Organization of the Gestoda. 425 



genesis of the larval forms known as Sporocysts and Redia?, 

 to a heterogamy which was for a long time believed to be 

 alternation of generations ; in the case of the Cestodes, on the 

 other hand, it produced, owing to individualization of gemina- 

 tion and fission-products of certain developmental stages, 

 various more or less complicated forms of alternation of gene- 

 rations, the modifications of which receive their natural 

 explanation and interpretation in the present resumS. 



Both the budding on the wall of the Cysticercus-vesicle and 

 the constriction and liberation of segments of the strobila are 

 already foreshadowed in the development of the Distomap, — 

 the former in the budding-power possessed by certain sporo- 

 cysts (Leucochloridium) , and the latter in the regular separa- 

 tion between the body and tail of the Cercaria and in the 

 fission-phenomena presented by certain sporocysts (e. g. those 

 of Cercaria minuta) and redias (those of Cercaria echinata 

 and fulvopanctata) . The caudal appendage also, the primary 

 function of which is that of a motile organ, is to be regarded 

 as a portion of the body which is capable of individualization. 

 This results from the surprising discovery made many years 

 ago by Alex. Pagenstecher * in the case of Bucephalus, and 

 only recently confirmed and also established in many other 

 cases by Ercolani f, that the tail is capable of transforming 

 itself into a brood-producing fragment — that is, as it were, 

 into a sporocyst. This process also elucidates the contrast 

 between the caudal appendage of the Cysticercoid and the 

 Cysticercus-vesicle and the invaginated neck or body of the 

 scolex, exhibiting the latter in the light of a further section 

 of the body of the worm, which, before the formation of pro- 

 glottids sets in, perhaps regularly separates itself from the 

 foremost portion representing the true head, and morpho- 

 logically is by no means so very different from the tail. 



Presuming it to be a legitimate and well-grounded assump- 

 tion, owing to the ensemble of the facts, that just as the 

 innumerable parasitic Copepoda, which present such manifold 

 variations and often such grotesque shapes, have been deve- 

 loped from free-swimming Crustacea, so also the intestinal 

 worms, through adaptation to a parasitic mode of life and the 

 conditions of existence modified thereby, have arisen from 

 free-living worm-forms ; then, with regard to the Platy- 

 helminthes, no doubt can exist that it was the Planarians — 

 so closely allied to the Trematodes — to which they owe their 



* Alex. Pagenstecher, " Trematodenlarven und Trematoden " (with 

 six plates), Helniinthologischer Beitrag : Heidelberg, 1857. 



f G. B. Ercolani, ' Nuove ricerche sulla storia genetica dei Treinatodi,' 

 Tom. i. 1881, and Tom. ii. 1882. 



