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the time of Leeuwenhoek to the present, no group of Crustacea has excited more interest, or been more studied by naturalists, than the genus Daphnia, or, perhaps, to speak more precisely, the Family Daphniidæ. So far, however, from exhausting the subject, these investigations have, in the present case, as in all other matters of science, opened out a still more interesting field for future labours, and suggested more questions than they have answered.

In the present memoir, Prof. Leydig gives a description of all the known species, and an interesting essay on their anatomy and embryology, the whole being illustrated by excellent figures. We are also made acquainted, for the first time, with several new species, and especially with a very curious new form, called by Prof. Leydig, Bythotrephes longimanus, and found by him in the stomach of Coregonus Wartmanni, of which, indeed, it appears in some localities to form the chief nourishment. And yet, though it must be so abundant, Prof. Leydig was unable to obtain a single specimen from the lake itself; but as the Coregonus is generally found at considerable depths, he infers that perhaps the Bythotrephes also seldom comes to the surface. The female only has as yet been discovered. The animal has the large antennæ, and the large eye as in Polyphemus, but the latter has longer lenses. The receptacle for the eggs is almost spherical, and so distinctly separated from the rest of the body, that it much resembles the abdomen of some spiders. There are four pairs of legs: the first is very much elongated, double as long as the rest, and five-jointed; the other three are not unlike those of Polyphemus. The abdomen also is very remarkable, and ends in a spine, half as long again as the rest of the body; and which, with the long anterior legs, the large eye, and the spherical receptacle, gives the whole animal a most peculiar appearance.

Since I have [had the advantage of reading Professor Leydig's work, I have had no opportunity of again examining the animals, and am therefore not now in a position to offer any new observations of my own with reference to those points as to which we differ; it seemed to me, however, that it was right to put my name to this review, because it will be necessary for me to refer more than once to my paper on Daphnia (Phil. Trans. 1857); and because, while fully admitting the great value and interest of the present work, I shall still be compelled to differ from Prof. Leydig on one or two important points.

Prof. Leydig adheres to the opinion that the agamic eggs of Articulata are internal buds, or as he would perhaps prefer to call them "germs" (Keime), and not to be confounded with true eggs; although he admits that in the Polyzoa and Rotatoria the so-called buds agree