Fig. [70].—Later stage in the development of the “Sacculina interna,” × 2. b, Body of Sacculina; c.t, “central tumour”; d.i, d.s, inferior and superior diverticula of alimentary canal of host; o, opening of perivisceral cavity of Sacculina; r, its roots.
Fig. [71].—Fourteen Cypris larvae fixed round the mantle-opening (o) of a young Sacculina externa, × 20.
The Rhizocephala are hermaphrodite with the possible exception of Sylon, which appears to be female and perhaps parthenogenetic, no male having been seen; but unlike most other hermaphrodite Cirripedes, they reproduce by a continual round of self-fertilisation. This is the more remarkable in that the vestiges of what appears to be a male sex are still found in Sacculina and Peltogaster; certain of the Cypris larvae in these genera, instead of fixing on and inoculating other crabs, become attached round the mantle-openings of young parasites of the same species as themselves, which have recently attained to the exterior of their hosts (Fig. [71]). These larvae, which remind us of the complemental males in Scalpellum, etc., never produce spermatozoa, but rapidly degenerate where they are fixed, and appear never to play any rôle in the reproduction of their species. The nature of this remarkable phenomenon, together with the sexual condition of the Cirripedes in general, will be discussed in the next section.
Much remains to be elucidated in the life-histories of these curious animals, and it seems probable that intermediate stages may exist, showing us how the extreme discontinuity of development has been reached. Suggestive in this respect is the newly discovered parasite of the Isopod, Calathura, which the author has named Duplorbis calathurae.[[73]] This animal does not appear to possess a root-system, but is attached to its host by a tube which passes right through the mesentery and opens into the mantle-cavity of the parasite. It may be suggested that this tube corresponds to the stalk of the normal Cirripede, but its exact mode of formation would certainly throw much light on the question of Rhizocephalan development.
Phenomena of Growth and Sex in the Crustacea.
In the foregoing account of the Cirripedia we have met with certain peculiar sexual relations in which closely allied species exhibit marked differences in regard to the distribution of the qualities of sex among their individuals; we have seen that the majority of species are hermaphrodite, unlike most Crustacea which, with the other exception of the parasitic Isopoda, are normally dioecious; and that in some species complemental males exist side by side with the hermaphrodites, while, in yet others, the individuals are either females or dwarf males.
Before examining the causes of these conditions, it will be opportune to consider a number of facts which throw light on the question of sex and hermaphroditism in general. We may then return to the discussion of the hermaphroditism found in particular in the Cirripedia and Isopoda.
Parasitic Castration.—Giard[[74]] was the first to observe that a number of parasites exert a remarkable influence on the sexual characters of their host, such that the generative glands become reduced, or may completely degenerate, while the secondary sexual characters become materially altered. This was proved to occur in the most widely different hosts, affected by the most widely different parasites (e.g. Crustacea, Insecta, Worms). Moreover, it was apparent that the affection does not consist in the parasite merely destroying the generative organs, with which it often does not come into contact, but rather in the general disturbance of the metabolism set up by its presence.