Till the further development of the infusoriform embryo is known it is not possible to arrive at a definite conclusion as to the affinities of this strange parasite. Van Beneden is anxious to form it, on account of its simple organization, into a group between the Protozoa and the Metazoa. It appears however very possible that the simplicity of its organization is the result of a parasitic existence; a view which receives confirmation from the common occurrence of the process of endogenous cell formation in the axial hypoblast cell. It has been clearly shewn by Strasburger that endogenous cell formation is secondarily derived from cell division; so that the occurrence of this process in Dicyema probably indicates that the hypoblast was primitively multicellular. It is not improbable that the enigmatical infusoriform embryo may develop into a sexual form, the progeny of which are destined to complete the cycle of development by becoming again parasitic in the renal organ of a Cephalopod. BIBLIOGRAPHY. (117) E. van Beneden. "Recherches sur les Dicyemides." _Bull. d. l'Académie roy. de Belgique_, 2e sér. T. XLI. No. 6 and T. XLII. No. 7, 1876. _Vide_ this paper for a full account of the literature. (118) A. Kölliker. _Ueber Dicyema paradoxum den Schmarotzer der Venenanhänge der Cephalopoden._ (119) Aug. Krohn. "Ueb. d. Vorkommen von Entozoen, etc." _Froriep Notizen_, VII. 1839. ORTHONECTIDÆ. A number of minute parasites infesting various Nemertines, Turbellarians, and Ophiuroids have recently been studied by Giard and Metschnikoff, the former of whom has placed them in a special group which he calls the Orthonectidæ. They were first discovered by W. C. McIntosh. In the adult state they are[66] (Metschnikoff) somewhat pear-shaped bodies formed of a kind of plasmodium of cells with irregular lobate processes. In the interior of this body are eggs in all stages of development. In the type observed by Metschnikoff (Intoshia gigas) the ova undergo a regular segmentation, resulting in the formation of a blastosphere in which an inner layer is subsequently formed by delamination. A smaller and a larger kind of embryo are formed; but all the embryos in each female belong to one type. The larger become females and the smaller males. [66] This at any rate holds true for the type investigated by Metschnikoff. The full history of other forms is not yet known. The female embryos are ovoid. The outer layer of cells or epiblast becomes ciliated, and divided into nine segments, of which the second is marked off from the remainder by the absence of cilia, and by being provided with refractive corpuscles. The inner layer which surrounds a central cavity, and might be supposed to be the hypoblast, becomes according to Metschnikoff converted into ova.