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. [Illustration: Fig. 98. Pollicipes Mitella.] 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. [Illustration: Fig. 99. Rotiferia; Brachionus Urceolaris; largely magnified. ]