We have said that two kinds of cells represent reproduction, namely, sperm and germ-cells. These may be furnished by different individuals, or both may be found in one. When both are found in the same individual, the parent is said to be a natural hermaphrodite. A perfect hermaphrodite possesses the attributes of both male and female—uniting both sexes in one individual. Natural hermaphroditic reproduction occurs only among inferior classes of animals, and naturalists inform us that there are a greater number of these than of the more perfect varieties. These are found low in the scale of animal organization, and one individual is able to propagate the species. In the oyster and ascidians no organs can be detected in the male, but in the female they are developed. Polyps, sponges, and cystic entozoa, may also be included among hermaphrodites.
It is only very low organisms indeed in which it is a matter of indifference whether the united sperm-cells and germ-cells are those of the same individual, or those of different individuals. In more elaborate structures and highly organized beings, the essential thing in fertilization is the union of these cells specially endowed by different bodies, the unlikeness of derivation in these united reproductive centers being the desideratum for perpetuating life and power.
In other classes, as entozoa, there appear to be special provisions whereby the sperm-cells and germ-cells may be united; i.e., the male organs are developed and so disposed as to fecundate the ova of the same individual. Sexual and non-sexual modes of reproduction are illustrated by that well-defined group of marine invertebrate animals, called cirripedia Fig. 98 represents one of this genus.
Some of these are not only capable of self-impregnation, but likewise have what are called complemental males attache to some of the hermaphrodites. In the whole animal kingdom, it may be doubted if there exists another such class of rudimentary creatures as the parasitic males, who possess neither mouth, stomach, thorax, nor abdomen. After exerting a peculiar sexual influence, they soon die and drop off; so that in this class of animals may be found the sexual distinctions of male, female, and perfect hermaphrodites.
There is a class of wheel-animalcules termed rotifera, of which the revolving volvox is one example. They have acquired this name on account of the apparent rotation of the disc-like organs which surround their mouths and are covered with cilia, or little hairs. They are minute creatures, and can best be viewed with a microscope, although the larger forms may be seen without such assistance. They are widely diffused on the surface of the earth, inhabit lakes as well as the ocean, and are found in cold, temperate, and tropical climates. The rotifera were once supposed to be hermaphrodites, but the existence of sexes in one species has been clearly established. The male, however, is much smaller, and far less developed than the female. In some of these species, germ-cells, or eggs, are found, which do not require fecundation for reproduction or development, so that they belong to the non-sexual class.
The third variety of hermaphrodites embraces those animals in which the male organs are so disposed as not to fecundate the ova of the same body, but require the co-operation of two individuals, notwithstanding the co-existence in each of the organs of both sexes. Each in turn impregnates the other. The common leech, earth-worm, and snail, propagate in this manner.
Unnatural hermaphrodism is characteristic of insects and crustaceans, in which the whole body indicates a neutral character, tending to exhibit the peculiarities of male or female, in proportion to the kind of sexual organs which predominates. Half of the body may be occupied by male, the other half by female organs, and each half reflects its peculiar sexual characteristics. Some butterflies are dimidiate hermaphrodites; i.e. one side of the body has the form and color of the male, the other the form and color of the female. The wings show by their color and appearance these sexual distinctions. The stag-beetle is also an example. We have accounts of dimidiate hermaphrodite lobster, male in one half and female in the other half of the body.