[Illustration: FIG. I.--Appendicularia, seen from the left side, _m_, mouth; _k_, gill intestine; _o_, oesophagus; _v_, stomach; _a_, anus; _n_, nerve ganglia (upper throat-knots); _g_, ear vesicle; _f_, ciliated groove under the gill; _h_, heart; _e_, ovary; _c_, notochord; _s_, tail.--_Haeckel._] [Illustration: FIG. II.--Represents Sack Worms (Himatega), and is the structure of an Ascidian, seen from the left. _sb_, gill-sac; _v_, stomach; _i_, large intestine; _c_, heart; _t_, testes; _vd_, seed duct; _o_, ovary; _o'_, matured eggs in the body cavity. After _Milne-Edwards_.] [Illustration: FIG. III.--Represents the ACRANIA SERIES. Lancelet (Amhioxus Lanceolatus), twice the actual size, seen from the left. _a_, mouth-opening, surrounded by cilia; _b_, anal-opening; _c_, ventral-opening (Porus abdominalis); _d_, gill-body; _e_, stomach; _f_, liver-coecum; _g_, large intestine; _h_, coelum; _i_, notochord (under it the aorta); _k_, arches of the aorta; _l_, main gill-artery; _m_, swellings on its branches; _n_, hollow vein; _o_, intestinal vein.--_Haeckel._] [Illustration: FIG. I.--Represents the MONORHINA SERIES. Lamprey (Petromyzon Americanus) from the Atlantic--_Orton._] [Illustration: FIG. II.--Represents the Selachii. Shark (Carcharias vulgaris) from the Atlantic--_Orton._] [Illustration: FIG. III.--Represents the Mud-fish (Dipneusta). Lepidosiren annecteus, one-fourth natural size; African rivers.--_Orton._ Form a link between typical fishes and the Amphibians.] At this stage, most probably, the separation of the two sexes began. The simpler and most ancient form of sexual propagation is through double-sexed individuals (hermaphroditismus). It occurs in the great majority of plants, but only in a minority of animals; for example, in the garden-snails, leeches, earth-worms and many other worms. Every single individual among hermaphrodites produces within itself materials of both sexes--egg and sperm. In most of the higher plants every blossom contains both the male organs (stamen and anther) and the female organs (style and germ). Every garden-snail produces in one part of its sexual gland eggs, and in another sperm. Many hermaphrodites can fructify themselves; in others, however, copulation and reciprocal fructification of both hermaphrodites are necessary for causing the development of the eggs. This latter case is evidently a transition to sexual separation (gonoehorismus). Out of the members of the last group arose animals with skulls or craniata, having round mouths, and which are divided into hags and lampreys. The hags (myxinoides) have long cylindrical worm-like bodies. The lampreys (petromyxontes) includes those well known "nine eyes" common at the seaside.