DISTOMID^. " 31 



by nourislinient or not. In particular, while in this stage, the 

 different kinds of hooks for migration purposes, make their appear- 

 ance, always, without doubt, after the shedding of the skin. Other 

 trematodes pass through this tail-less sexually immature stage 

 without any cyst. I have not yet seen any larval-trematode forms 

 which had been produced in sporocysts or redise without ap- 

 pendages ; they appear to occur, nevertheless. 



" (k.) — As the larvse exist only in a few hosts — and most of 

 them dwell only in one species of animal — so, also, the continued 

 progress towards sexual maturity only succeeds in the case of cer- 

 tain well-defined larval organisms ; but the digestion of the cysts 

 and liberation of the larvas may be accomphshed in various animals. 



" (L) — The armed cercarise appear to be larvae of the spine- 

 covered distomes of amphibia ; for, as examples, the Gercaria 

 ornata becomes transformed into Bistom,a clavigerum, and G. armata 

 into Distoma endolobum ; the Dist. dujoUcatum and Gere, diplo- 

 cotylea are, apparently, the juvenile forms of Bist. cygnoides and 

 Am/pJiistoma subclavakim. The Dist. echiniferum of Paludina could 

 neither be advanced in development in the frog or duck, nor could 

 all the other larv^ which I subjected to experiment be developed 

 either in the green or brown frogs. 



" (m.) — ^When young trematodes arrive at the right place for 

 their maturation, then the male generative structures develop 

 before the female organs, and in the subsequent excess of egg pro- 

 duction the form and structure of the animal becomes obliterated. 

 A copulatory act is not described as certain, but some speak of 

 such an arrangement, while others are in favour of the view of 

 self-impregnation . * 



* I beg to call Dr. Pagenstecher's attention to the fact, that I have observed an 

 actual sexual congress in the case of Distoma conjunctum. This was first recorded in my 

 third paper, published in the " Linnean Transactions," vol. xxiii. p. 350, and I have else- 

 where alluded to the circumstance, as well as in the present work (p. 22). I do not be- 

 lieve any one has ever witnessed a process of self-impregnation in the trematode entozoa 

 or in their allies. Notwithstanding Leuckart's representations respecting Tania echino- 

 coccus, I am not certain that this process is ordinarily accomplished in the Oestoda. 



T. S. C. 



