The foregoing facts indicate unmistakably that the males of a number of Crustacea under certain metabolic conditions, _i.e._ when a stage of active growth as opposed to a stage of reproductive activity is initiated, alter their sexual constitution in such a way that the latent female characteristics are developed, and the organism appears as a partial hermaphrodite. In the preceding paragraph we saw that the males of a number of animals, especially Crustacea, react to the metabolic disturbance set up by the presence of a parasite in exactly the same way, _i.e._ by developing into partial or total hermaphrodites. From these two converging bodies of facts we may conclude, firstly, that sex and metabolism are two closely connected phenomena; and, secondly, that the male sex is especially liable to assume hermaphrodite characters whenever its metabolic requirements are conservative, assimilatory, or in a preponderating degree anabolic, as when a phase of active growth is initiated, or the drain on the system, due to the presence of a parasite, is to be made good. =Normal Hermaphroditism in Cirripedia and Isopoda Epicarida.= The above-mentioned groups contain the only normally hermaphrodite Crustacea, and since they are in most respects highly specialised, we may be certain that they have been secondarily derived from dioecious ancestors. They both lead a sessile or parasitic life, and it is noteworthy that this habit is often associated with hermaphroditism, _e.g._ in Tunicates. A sessile or parasitic mode of life is one in which the metabolic functions are vegetative and assimilatory rather than actively kinetic or metabolic. It is in this state that we have seen the males of a number of Crustacea taking on a temporary or partial hermaphroditism. We may, therefore, inquire, whether in these cases of normal hermaphroditism there is any evidence to show that here too the hermaphroditism has been acquired by the male sex as a response to the change in the metabolic conditions. In the parasitic Isopoda Epicarida (see pp. 129–136) the hermaphroditism is of a very simple kind; all the individuals are at first males, whose function it is to fix on and fertilise the adult parasites. These subsequently develop into females which are in their turn cross-fertilised by the young larvae derived from a previous generation. All the individuals being alike, it seems probable that they have been derived from one sex, and the general nature of hermaphroditism deduced above may lead us to suppose that that sex was originally male, the female having been suppressed. In certain Cirripedia, _e.g._ most species of _Scalpellum_, there exist, besides the hermaphrodite individuals, complemental males, so that here a superficial conclusion might be drawn that the hermaphrodites represent the female sex. But if we can suggest that the complemental males are in reality similar in derivation to the hermaphrodite individuals, we shall