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Rh to designate the peculiar semi-gelatinous substance composing the body of these simple organisms. In the year 1835, it was announced by this observer, that the animal inhabitant of certain microscopic shells termed Foraminifera by D'Orbigny, though by him placed among the Cephalopoda, agreed essentially in organization with the Amæba, or Proteus animalcule and other fresh-water forms allied thereto. It then became desirable to separate these, as a group, from the true Infusoria; and accordingly they were soon united into a class, under the name of Rhizopoda, a term first introduced by M. Dujardin, but employed by him in not quite so extended a signification.

At length, in the year 1845, Von Siebold founded the modern sub-kingdom, Protozoa, which he divided into two classes, Infusoria and Rhizopoda. No mention, however, is made by him of the Sponges.

Previously, in the year 1838, the name of Polycystina had been given by Ehrenberg to a group of microscopic siliceous-shelled creatures, evidently allied to the Foraminifera. Ehrenberg, it is true, entertained very peculiar views of their affinities, and even sought to place them in the neighbourhood of his Bryozoa.

So early as 1828, Dufour had established the genus Gregarina for the reception of certain minute parasitic organisms found by him in the bodies of insects. In the year 1841, J. Müller described, under the title of Psorospermiæ, the contents of small rounded cysts occurring in the cellular tissue of the muscles of a young pike's eye. Ten years afterwards, the existence of a curious relationship between these Psorospermiæ and the Gregarinæ of Dufour was ingeniously demonstrated by Leydig. Doubts soon arose as to the position of these parasitic forms. Creplin had suspicions of their vegetable nature; the hasty observation of some phenomena in their development, which simulated the conjugation of the lower Algæ, appeared, at first, to countenance this conjecture. Bruch, Leydig, and a number of other observers, regarded them as Helminthes, or, at least, as transitory stages in the life-history of these animals. But Kölliker, in a paper of great value, brought forward arguments which went far to prove, (1), that the Gregarinæ were true animals; (2), that no good evidence had been produced to