Encouraged by the experiences and determinations of Pagenstecher, Filippi, Wagener, Leuckart, and others, I sought for the intermediate hosts amongst fresh-water mollusks and small crustacea. Failing of success in these, it occurred to me that the larvæ of _Bilharzia_ might normally reside in fluviatile or even in marine fishes. This latter idea seems also to have struck Dr Aitken. In an appendix to his ‘Report to the Army Medical Department for 1868,’ dated from Netley, Nov., 1869, he gives a figure of a nurse-form, which he terms a cercaria, from the tail of a haddock--suggesting for _Bilharzia_ some genetic relation. Dr Aitken also extends his views in reference to certain larval trematodes alleged to have been found in the so-called Delhi boils and Lahore sores. These parasitic forms have, however, been shown by Dr Joseph Fleming to be nothing more than altered hair-bulbs (‘Army Med. Reports,’ 1868-69). In regard to the flukes from the haddock, I have satisfied myself that these immature trematodes from the nerves of the cod-tribe can have no genetic relation with _Bilharzia_; and I think it due to Dr Maddox to say that I accept his conclusion respecting them. In his paper (‘Micros. Trans.,’ vol. xv, 1867, p. 87) he offers strong proof that the so-called _Distoma neuronaii Monroii_ of the haddock (_Morrhua æglefinus_) is the juvenile condition of _Gasterostoma gracilescens_ of the angler (_Lophius piscatorius_). I am sorry to have to state that all my experiments proved negative. I tried to induce the ciliated embryos to enter the bodies of a variety of animals, such as Gammari, Dipterous larvæ, Entomostraca, Lymnæi, Paludinæ, different species of Planorbis, and other mollusks; but neither in these, nor in Sticklebacks, Roach, Gudgeon, or Carp, did they seem inclined to take up their abode.