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Show 498 DR. P. SONSINO ON DISTOMES. [May 16, less than three different orders of Mammals, undoubtedly on account of the food that they may partake of in common. DISTOMTJM OVO-CAUDATUM, Vulpian. Bistomum ovo-caudatum, both of Rana esculenta and of R. tempo-raria, has been described since 1859, by Vulpian, as living in the mouths of these animals. Vulpian gave an interesting account of this worm and has not failed to put in evidence the important character of the presence of a basal filament in the egg,-a character very rarely to be seen in the digenetic Trematodes, but frequently observed in monogenetic or ectoparasitic Trematodes. Recently Creutzburg has published an interesting study of the life-history of the same worm, which according to him in its larval form is the famous Gercaria cgstophora, Wagener, which was once referred by Willemoes-Suhm to B. lanceolatum, Mehlis. According to Creutzburg, the intermediary host of B. ovo-caudatum, a Planorbis, would become infested not by active introduction of the embryo into its body, the embryo being unable to swim in water, but by eating the eggs with the contained embryo, which eggs are found in the faxes of frogs. In a similar way the final host would be infested by eating C. cystophora contained in its nurse within the tissues of the mollusk. But as yet Creutzburg has not succeeded in rearing the adult worm in the mouth of a frog, by introducing the tissues of Planorbis containing C. cystophora. I a m certainly not prepared to criticise Creutzburg's work, which is only preliminary; but I wish to point out some facts observed by myself, which are not in accordance with those noticed either by Creutzburg or Vulpian, respecting the worm in question. I have had good opportunities to observe and study B. ovo-caudatum, which I have found rather frequent in both Rana esculenta and R. temporaria in the neighbourhood of Pisa. I may now draw the following conclusions as to the result of m y observations, of which I gave a full account in the ' Monitore Zoologico' of last Aprd:-(1) B. ovo-caudatum takes up its abode not only in the mouth, but also in the stomach and the upper part of the intestines. It is perhaps more frequent in the stomach than in the mouth, and it is possible that its occurrence in the mouth is merely accidental, and the result of an emigration, occasioned by the distressed condition of the worm on the death of the host. (2) The basal filament is from 4 to 6 times as long, or even longer, than the length of the body of the egg. If previous observers have given a shorter length, it is probably in consequence of their having seen only a truncated filament, or of not having properly focussed the microscope so as to see the entire length of the filament, which gradually thins towards the end. (3) The embryo is not only armed with a crown of spines (bdtonnets) on its anterior extremity, but evidently possesses even cilia. Whether these cilia aid it to swim in water when it issues from the egg, spontaneously and completely mature, or are simply for locomotion in the intestines of Planorbis, I a m not prepared to say, not having seen the |