Hermaphroditism, the combination of the essential male and female functions and structures in one organism, as in most flowering plants, or in many lower animals, such as earthworm, leech, or snail. The name is derived from the fable of the union into one of the bodies of Hermaphroditus, son of Hermes and Aphrodite, and the nymph Salmacis (see Ovid's Metamorphoses, iv. 347). The combination of two sexes in one occurs, however, in various degrees, the bisexuality being sometimes very intimate, and in other cases only superficial. (a) It is probable that many animals—e.g. frogs, which are unisexual in adult life—pass through a period of embryonic hermaphroditism, early nutrition having much to do with the more or less complete predominance of one sex over the other. (b) Among fishes and amphibians and elsewhere, casual or abnormal hermaphroditism is not infrequent, the animal having for instance an ovary on one side and a testis on the other. (c) In other cases only one organ is developed, and one sex emphatically predominates in the organism, not, however, without hints of the other. This partial hermaphroditism is usually an exception, as when a butterfly has its wings coloured like those of the female on one side, like those of the male on the other. Frogs and toads also illustrate curious combinations, which do not, however, conflict with the predominance of the egg-producing or the sperm-producing function as the case may be. (d) An apparent, but in reality false hermaphroditism may result in the higher animals where, by malformation or rudimentary development of the external reproductive organs, a mammal in reality quite female may look like a male, or vice versa. (e) Normal adult hermaphroditism, where egg-producing and sperm-producing functions go on (usually at different times), is rare among higher animals—occurring in Chryphrys and Serranus among fishes, in the hagfish Myxine, and in all the Tunicata. It is, however, of frequent occurrence in the invertebrate series—among snails, bivalves, cirripedes, worm-types, cœlenterates, and sponges. It is most familiar in our common flowering plants, which are often called monoclinous or perfect.
Hermaphroditism may be more or less intimate. Thus, as an entire plant an Arum is hermaphrodite, with female flowers below and male flowers above; but the hermaphroditism is more intimate in a buttercup, where each flower bears male and female organs, or yet more intimate in an orchid, where stamens and carpels are united. So a leech, with ovaries quite distinct from the testes, is less intimately hermaphrodite than a snail, where within the same small organ both kinds of sex elements are produced.
The male and female elements, whether in phanerogam or invertebrate, are rarely, if ever, matured at the same time. Such a 'want of time-keeping' is called in botanical language dichogamy, and is one of the conditions which tend to prevent self-fertilisation. Protandrous dichogamy, where the stamens take the lead, is much commoner than protogynous dichogamy, where the carpels mature first. This is also true of animals, and is more marked when the hermaphroditism is intimate, as in snail or oyster. The hagfish seems to be predominantly male till it attains a certain size; and so in the curious thread-worm Angiostomum and in the crustacean Cymothoidæ the organs are first male and then after a while female. In the cirripedes and Myzostomata, the majority of which are bisexual, pigmy or supplemental males are in some cases associated with the hermaphrodites, or in the case of the barnacles (in which separate sexes sometimes occur) even with some of the females.
Alike in plants and in animals, though herma- phroditism is common, self-fertilisation is rare. It does occur in not a few common flowers, and in tapeworms, some flukes, and a few other animals, but is without doubt exceptional.
Hermaphroditism is commonest in sluggish animals (e.g. flat-worms, tardigrades, snails), or in fixed animals (e.g. sponges, corals, Polyzoa, bivalves, Tunicates), or in parasitic animals with a plethora of nutrition and little exertion (e.g. flukes, tapeworms, leeches, Myzostomata).
As to its origin, hermaphroditism is probably the lower, more primitive condition from which that of unisexuality has been in the majority of cases evolved. In alternating rhythms eggs and sperms were produced, gradually the areas of their respective formation were restricted, by-and-by one tendency predominated in the organism, and separate males and females were established. If embryonic hermaphroditism be, as some believe, of general occurrence, then most organisms recapitulate this evolution of separate sex in their individual life-history. If it be allowed that hermaphroditism was the primitive condition, then the cases now existing indicate either persistence or reversion. See EMBRYOLOGY, REPRODUCTION, SEX; and Geddes and Thomson, The Evolution of Sex (Lond. 1889). For aberrant hermaphroditism in human adults, see Todd and Bowman's Cyclop. of Anat. and Physiol., vol. ii.