lay eggs, out of which Paramecia were born, which underwent all the changes these animals are known to undergo up to the time of their contraction into a chrysalis state; while the Opalina is hatched from Distoma's eggs. I shall publish the details of these observations on another occasion. But if it can be shown that two such types as Paramecium and Opalina are the progeny of worms, it seems to me to follow that all the Enterodela, with the exception of the Vorticellidæ, must be considered as the embryonic condition of that host of worms, both parasitic and free, the metamorphosis of which is still unstudied. In this connexion I might further remark, that the time is not long past when Cercaria was also considered as belonging to the class of Infusoria, though at present no one doubts that it belongs to the cycle of Distoma; and the only link in the metamorphosis of that genus which was not known is now supplied, since, as I have stated above, the embryo which is hatched from the egg laid by the perfect Distoma is found to be an Opalina."
"All this leads to the conclusion that a division of the animal kingdom to be called Protozoa, differing from all other animals in producing no eggs, does not exist in nature; and that the beings which have been referred to it have now to be divided, and scattered, partly among plants, in the class of Algæ, and partly among animals, in the classes of Acephala, (Vorticellæ,) of Worms, (Paramecium and Opalina,) and of Crustacea, (Rotifera); the Vorticellæ being genuine Bryozoa, and therefore Acephalous Mollusks; while the beautiful investigations of Dana and Leydig have proved the Rotifera to be genuine Crustacea, and not worms."
In these passages it will be observed that much which cannot be accepted is blended with statements of facts long since admitted to be true, and thus a certain degree of plausibility conferred upon the whole.
Few, indeed, will deny that the Desmidia, Volvocinæ, and several other organisms referred by Ehrenberg to his Polygastrica, are true Algæ. That many of the Enterodela may yet prove to be embryonic forms seems also highly probable. To conclude, however, that all the Infusoria, with the exception of the Vorticellidæ, may thus readily be disposed of, appears at best a somewhat hasty mode of removing difficulties, the solution of which must depend on a long series of patiently conducted embryological inquiries. Because two or three forms of supposed Infusoria are shown to be stages of development in the life-history of certain worms, it by no means follows that all remaining Infusoria are to be likewise so regarded. Nor does the careful examination of such a genus as Pedicellina, which of all the Bryozoa most closely approaches Vorticella in form, strengthen the opinion entertained by Prof. Agassiz of their mutual relationship, but is rather decidedly opposed to it.
The assertion that Rhizopoda have not yet found "a place generally acknowledged as expressing their true affinities," would scarcely lead the reader to suppose that Brown, Carus, Gegenbaur, Siebold, Vogt, Van Beneden, Gervais, and many other zoologists, agree in referring them to the type of Protozoa. And he who reads with care the memoirs of Carpenter and Williamson on the shelly structure of the Foraminifera, will scarcely be disposed to call in question their animal nature, or liken the highly complex frame-work of Peneroplis or Polystomella to the stony frond of a Melobesia, in every essential respect so different. Still less does the sarcode substance of the Rhizopods and Sponges resemble, in its vital endowments, the viscid contents of the vesicles of Fuci. Lastly, it is incorrect to state that the Protozoa, as a group, are distinguished from other animals in producing no eggs, since, even
