142 



render it necessary to consider this animal as a true Tre- 

 matod ; but it is a Trematod presenting the external form of 

 a Nematod, which attains more than a metre of length, and 

 which lives in quite exceptional conditions. 



These dij^enetic Trematods whose embryo is deprived of 

 cilia cannot be considered as forming the transition to the 

 Cestoids ; and we must not suppose that in these embryos 

 with prickles we have the homologue of the six-hooked 

 embryo of the Band-worms. From the beautiful observa- 

 tions of Schubart and of Knoch' we know that in the 

 Bothriocephalids the six-hooked embryo is not the first 

 embryonic form, but that this embryo succeeds a ciliated 

 form, from which it arises by metagenesis. 



I have pointed out in my memoir " On the Composition 

 and the Significance of the Egg " that in the Tseniadse {Tmnia 

 bacillaris) a membrane is formed corresponding to this 

 ciliated investment of the Bothriocephali ; but it is constantly 

 free from vibratile cilia. The hexacanth embryo is then both 

 in the one and the other a secondary embryonic form, born by 

 metagenesis from an embryo, sometimes ciliated, sometimes 

 free from vibratile cilia. The ciliated embryo of the digenetic 

 Trematods corresponds to the ciliated form of the Bothrio- 

 cephali ; and it is the sporocyst which corresj^onds to the 

 hexacanth embryo of the Cestoids. 



The embryo of Distoma tereticolle, of Monostoma filum, 

 oi Neniatobothrium, is evidently the homologue of the ciliated 

 embryo of the other Distomata, and consequently, in spite of 

 their crown of booklets, they represent in no respect what- 

 ever the hexacanth embryo of the Cestoids. 



The following table sets forth the remarkable parallelism 

 which one can establish between Trematods and Cestoids as 

 regards their development : 



r Monogenesis . . . . = Caryophyllae^is. 



es 1 s < DiggQggjg^ — "pijg grst embryonic C Ciliate = Bothriocephalus. 

 [^ form is \ Non-ciliate = Tceniada. 



r Monogenesis .... = Ectoparasites. 



Trematods \ f Ciliate =: Bistoma, Mono- 



-p.. . mi /. 1 1 -I stoma, Gasfero- 



1 Digeuesis.-The first embryonic . ^^ ^^_ 



form is 



^ Non-ciliate =: Distoma te- 

 reticolle, 

 Monostoma 

 filum, Nema- 

 tobothrium. 



' Knoch, ' Naturgescliichte des breiten Bandwurms.' St. Petersburg 

 1862, 4to. 



