"oumai.Tay 'iS'] I^oijol Micwscopieal Society. 233 



had discovered a new species. My search for either a description or 

 drawing was unavaihng, until Dr. Baird directed my attention to 

 Dr. Nitzsch's small volume, in which I found a very badly-drawn 

 specimen of Cercaria furcaia, somewhat resembling mine. It is 

 there grouped among some strange companions, and seemingly 

 placed by this author among infusorial animalcules. It is quite 

 unnecessary to dwell upon the remarkable metamorphoses which 

 trematode Entozoa undergo during development into flukes ; but as 

 every species of animal appears to be liable to be infested by a fluke, 

 it will, I am sure, be considered not out of place if I attempt to call 

 more particular attention to a point or two in the history of this 

 cercaria which I think has hitherto not been dwelt upon with 

 sufiicient care. 



Early writers on the parasites of molluscs look upon them as a 

 disease, and this view is held to the present day. It is, however, 

 somewhat remarkable that the hundreds of these little animals which 

 are seen to cluster around the body of the strongest and healthiest 

 snails, produce no apparent discomfort to their nurses. They like- 

 wise take possession of the smaller and younger snails, whose growth 

 is in no way impeded, nor health derauged ; but after having com- 

 pleted this phase of existence, they are weaned without difiiculty, 

 and only wait the opportunity to begin a new life in the stomach of 

 a higher order of animals. AVhen the snail is kept in a confined 

 space, and without a sufiicient supply of vegetable food, the cercariae 

 slip from their hiding place, swim about freely, and are soon lost sight 

 of ; this would scarcely be the case if they sucked their nurse and 

 depended upon her for nourishment. It is questionable, then, whether 

 it is right to regard their presence as a condition of disease ; possibly, 

 they bear no other relation to an abnormal condition than the spores 

 of the mushroom do to the stomach of the horse, through which 

 viscus they must pass before they are in a fit state to germinate. 

 Eeaumur took cercariae to be " mites ; " Miiller, " worms." The 

 latter called them " vibrio Malleus ; " Bory " Histrionella Jissa ;" 

 and Steenstrup first noticed their transformation to Distoma. 



Among the earhest writers who seem to have understood these 

 curious hair- like worms, was Dr. Christian Ludwig Nitzsch, who in 

 1817 published a smaU monogram, entitled ' Bertrag zur Infusorien- 

 kuude oder Naturleschreibung der Zerkarien und Bazillarien.' Among 

 other badly-executed drawings of infusorial animalcules is one which 

 is intended to represent Cercaria furcata. Its peculiar forked-like 

 tail is almost sure to induce anyone who sees it for the first time, to 

 think that it should be properly placed among Eotifers. V. Baer in 

 an excellent paper* enters more fully into the peculiarities of this 

 species, and of the various forms it assumes. He discovered cercariae 

 on Paludina vivipora. Diesing, in his ' Systema Helminthum,' 



* Nov. Act. Nat. Cor. xiii. 2. p. G27. 1827. 



