Page:Natural History Review (1861).djvu/39

Rh course, of a central layer of corium, surrounded on all sides, except at the line where it joins the body, by a chitinous envelope. When the animal is about to shed its skin, the corium secretes around itself, and between itself and the old chitinous layer, a new layer of chitine; and if an ephippium is being formed, the outer case is produced by the external part of the old layer of chitine, while the chitine which clothed the inner surface of the valve (inner, with reference to the receptacle, though still, of course, in reality, an external skin), is modified into the inner capsule of the ephippium, and the new layer of chitine takes, therefore, no share in this process.

As a proof that this view was correct, I cited the fact, that on removing the outer valve of a Daphnia, which was about to change its skin, the animal swam away, and the inner ephippial valve remained in the receptacle. M. Smitt seems to doubt whether there is not some mistake in this statement, since he says, "Il doive déjà étonner, que les œufs, la mère changeant de test, ne soient pas considérablement dérangés, et que la carapace nouvelle, qui les separe du test exterieur de l' éphippie, ne les emporte pas avec soi;" but I never said that the eggs were not disturbed; it requires some practice and much steadiness of hand to remove the old carapace, even from a large specimen, without injuring the animal; but I never said that it could be done without deranging the eggs.

I consider that the external and internal layers of the shell (both of which are formed of epidermis, and are secreted by the corium or chitinogenous layer which lies between them), constitute the outer and inner valve of the ephippium, but M. Smitt understands me to suppose that the inner layer of the ephippium is formed by the corium itself; and he adds, that in this case it is impossible to understand in what manner the new shell is produced. He suggests, indeed, that as the outer valve of the ephippium is formed of the old shell, so the inner valve may be formed by the new one; but he admits that this explanation can- not apply to those cases in which the Daphnia survives the removal of the ephippium.

I must admit that M. Smitt has conclusively disproved the theory which he supposes that I propounded; and I gladly take this opportunity of thanking him for the courteous manner in which he has done so. I trust, however, that, though the relation of the parts is somewhat intricate, the present description is sufficiently precise; and that, on a second examination, M. Smitt will feel justified in adopting the explanation which I have offered. M. Smitt has also made the curious observation, that in Daphnia sima the ephippium contains only one egg. This fact, if it hold good generally, and was not merely the result of accidental circumstances, is certainly very interesting. In D. Schœfferi, which alone I examined with much care, two ephippial eggs were always produced simultaneously, one in each ovary. Since reading M. Smitt's paper, I have provided myself with some specimens of D. sima; but have not yet succeeded in meeting with any ephippia.

Professor Leydig has observed the male organs in several species.