DISTOMIDiE. 31 



by nourislimeiit 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 rediae without ap- 

 pendages ; they appear to occur, nevertheless. 



" (k.) — As the larvae 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 organisnls ; but the digestion of the cysts 

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



" (/.) — The armed cercariae appear to be larv^ of the spine- 

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

 ornata becomes transformed into Distoma clavigerum, and G. armata 

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

 cotj/Iea are, apparently, the juvenile forms of Dist. cygnoides and 

 Amjphistoma subclavatum. The Dist. echiniferum of Paludina could 

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

 all the other larvae which I subjected to experiment be developed 

 either in the green or brown frogs. 



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

 their maturation, then the male generative structures develop 

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

 duction the form and structure of the animal becomes obhterated. 

 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 bes to call Dr. Pagenstecher's attention to tte 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 alUes. Notwithstanding Leuckart's representations respectiag T<sma echino- 

 coceus, I am not certain that this process is ordinarily accomphshed in the Cestoda. 



T. S. C. 



