_Natura non facit saltus_ is especially true as to the division of sexes among the nematodes. Between the true hermaphrodites and the true dioecious worms are found species in which the males gradually dwindle and become dependent on the female; this is to be seen in the _Sphoerulariæ_, among which the male is only an appendage to the female sex. We find here full evidence of the fact that the female is more important than the male, with regard to the preservation of the species. In some species the sexes differ but little, in others, the sexual differences become greater, and the male is only one third of the length of the female; but in some of them the disproportion is greater still. At the same time, we see nematodes whose males are attached to the females, so as only to form a single individual; in other cases, the male seems to disappear to such an extent, that we find nothing but the male organ in the female; indeed, there are instances of male worms, which, without changing their form, occupy the cavity of the matrix and, like the lernean crustaceans, are parasites of their females. The _Trichosomum crassicauda_ is an instance of this kind. Arrangements which would not have been suspected beforehand, are every day revealed, with respect to the conservation of species. We have recently learned from the works of Messrs. Malmgren and Ehlers, and later still, from those of Claparède, that in the same species we may find different males, producing different offspring. Messrs. Malmgren and Ehlers have opened this question by their persevering researches, 236 and Mons. Claparède expected to invalidate the results obtained by them by establishing himself at Naples, in order to devote himself to a new series of investigations. Contrary to his expectations, he arrived at the same conclusions, and announced that a nereid possesses, in one and the same species, two kinds of males and two sorts of females, and that these males differ from each other, not only in their manner of life but in their age, in the mode of formation of the spermatozoïds as well as in the form; that the females differ no less from each other than the males, and that each form is intended to provide, in its own manner, for the dissemination of the eggs. We see this realized in annelid worms known by the name of _Heteronereidæ_. Certain individuals of small size live on the surface of the water; others, evidently much larger, live at the bottom of the sea and behave quite differently. The eggs and the spermatozoïds proceeding from these two forms differ sensibly from one another, and the difference of form corresponds with that of origin. We see thus among some of them different males; among others different females: then eggs and spermatozoïds equally different in one and the same animal species.