,
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 rou