In these cases self-fertilization is often absolutely excluded; it may be physically impossible, and hermaphroditism therefore secures cross-fertilization in such cases just as effectively as if the sexes were separate. Similarly, in many hermaphrodite flowers, as we have already seen, the pollen is so constituted and so placed within the flower that it cannot of itself make its way to the stigma. In oysters, for instance, the young animal is male, and liberates into the water an enormous quantity of minute spermatozoa, and therewith fertilizes the older individuals, functioning only as females, which have grown upon the same bank. At a later stage of its development the oyster which was male becomes female, and produces only ova. This state of affairs, of which I shall shortly mention another case, has been called _temporary_ hermaphroditism. In this case not only is self-fertilization excluded, but close inbreeding also, since it is always a young generation functioning simultaneously as males that mingles with an older generation which has become female. It is quite otherwise with parasites which live singly within the body of a host: for these it was indispensably necessary that they should not only produce both kinds of germ-cells, but that they should unite the two kinds in fertilization, and they therefore possess the power of self-fertilization. Thus, in the urinary bladder of the frog, there occurs a flat-worm (_Polystomum integerrimum_) which possesses special organs for pairing with another individual, but which is also capable of self-fertilization when, as frequently occurs, it has no companion in its place of abode. But this self-fertilization is always liable to be interrupted by cross-fertilization, for not infrequently there are two, three, or even four such parasites within the bladder of a single frog. In the tape-worms, too, cross-fertilization is not excluded, for there are often two or more of these animals together in the intestine of a host at the same time. But even where there is only one, self-fertilization on the part of the joints, that is, the sexual individuals, is prevented, and by the same device, metaphorically speaking, as in the case of the oyster, for in each joint the male elements mature first and the female elements afterwards. In certain parasitic Isopods of the genus _Anilocra_ and related forms close inbreeding is prevented in the same way--by a difference in the period at which the two sets of gonads in the hermaphrodite individual become mature (dichogamy).