The condition of the middle-sized males may be looked upon as one of partial hermaphroditism, indications of the female state being found in the flattened chelae and in the reduced state of the testes. This interpretation is greatly strengthened by the state of affairs observed in the life-history of the male Sand-hoppers, Amphipods of the genus Orchestia.[[79]] In the young males of several species of this genus, at the time of year when they are not actively breeding, small ova are developed in the upper part of the testes of more than half of the male individuals, these ova being broken down and reabsorbed as the breeding season reaches its height. Nor is this phenomenon confined to this genus; in the males of a number of widely different Crustacea these small ova are found in the testes at certain periods of the life-history (e.g. Astacus[[80]]), when the animal is not breeding.
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 be in a position to claim that the hermaphrodite Cirripedes are similar to the Isopoda Epicarida, and have probably also been derived from the male sex. There is decided evidence pointing to this conclusion. In the first place, the complemental males of at least one species of Scalpellum, S. peronii, do show an incipient hermaphroditism[[81]] in the presence of small ova in their generative glands, which, however, never come to maturity.
The condition of the degenerate males in the Rhizocephala may also be interpreted in the same manner. These never pass beyond the Cypris stage of development, in which they resemble in detail the Cypris larvae of the ordinary hermaphrodite individuals, and they are quite useless in the propagation of their species.
It is more reasonable to suppose that these Cypris larvae, which fix on the mantle-openings of adult parasites, are in reality identical with the ordinary Cypris which infest crabs and develop into the hermaphrodites, than that they represent a whole male sex doomed beforehand to uselessness and degeneration. If we suppose that the Cirripedes have passed through a state of protandric hermaphroditism similar to that of the Isopoda Epicarida, it is plain that all the larvae must have originally possessed the instinct of first fixing on the adult parasites, and we may suppose that this instinct has been retained in the Rhizocephala, but is now only actually fulfilled by a certain proportion of the larvae, which, under existing circumstances, are useless and fail to develop further; while the rest of the larvae, not finding an adult parasite to fix upon, go straight on to infect their hosts and develop into the adult hermaphrodites.
The same explanation would apply to the complemental males in Scalpellum, etc., these individuals being also potential hermaphrodites, which are arrested in development, though not so completely as in the Rhizocephala, owing to the position they have taken up.
This theory throws light on another dark feature of Cirripede life-history, namely, the gregarious instinct. The associations of Cirripedes are not formed by a number of Cypris larvae fixing together on the same spot, but rather by the Cypris larvae seeking out adolescent individuals of their own species and fixing on or near them. Now, if we suppose that the Cirripedes have passed through a condition of protandric hermaphroditism similar to that of the Isopoda Epicarida, it is clear that a slight modification of the sexual instinct of the larvae would lead to the gregarious habit, while its retention in some individuals in its original form accounts for their finding their way to the mantles of adult individuals and developing into the so-called complemental males.
Certain Cirripedes, viz. certain species of Scalpellum and Ibla and all the Acrothoracica, are dioecious. It is impossible to decide at present whether these species retain the primitive dioecious condition of the ancestral Cirripedes, or whether they too have been derived from an hermaphrodite state, but in the present state of knowledge they hardly affect the validity of the theory that has been proposed to account for the nature of the complemental males and the hermaphrodite individuals.