lum); _m_, mouth;
_sd_, throat-epithelium; _sm_, throat-muscles; _d_, stomach-intestine;
_nc_, kidney-ducts; _nm_, opening of the kidneys; _au_, eye; _na_,
nose-pit. Fig. II, the same Gliding Worm, showing the remaining organs;
_g_, brain; _au_, eye; _na_, nose-pit; _n_, nerves; _h_, testes;
[male symbol], male opening; [female symbol], female opening; _e_,
ovary; _f_, ciliated outer-skin.--_Haeckel._]

[Illustration: FIG. III.--Represents Soft Worms (Scolecida) and is a
young Acorn Worm (Balanoglossus), after _Agassiz_. _r_, acorn-like
proboscis; _h_, collar; _k_, gill-openings and gill-arches of the
anterior intestine, in a long row, one behind the other, on each side;
_d_, digestive posterior intestine, filling the greater part of the body
cavity; _v_, intestinal vessel, lying between two parallel folds of the
skin; _a_, anus.]

Out of the planula, then, develops an exceedingly important animal
form--the gastrula (that is, larva with a stomach or intestine), which
resembles the planula, but differs essentially in the fact that it
encloses a cavity which opens to the outside by a mouth. The wall of the
progaster (primary stomach) consists of two layers of cells: an outer
layer of smaller ciliated cells (outer skin or ectoderm), and of an
inner layer of large non-ciliated cells (inner skin or entoderm). This
exceedingly important larval form, the "gastrula," makes its appearance
in the ontogenesis of all tribes of animals. These gastraeada must have
existed during the older primordial period, and they must have also
included the ancestors of man. A certain proof of this is furnished by
the amphioxus, which, in spite of its blood relationship to man, still
passes through the stage of the gastrula with a simple intestine and a
double intestinal wall.[16] By motion of the cilia or fringes of the
skin-layer, the gastraea swam freely about in the Laurentian ocean.

The development of the gastraea now deviated in two directions--one
branch of gastraeads gave up free locomotion, adhered to the bottom of
the sea, and thus, by adopting an adhesive mode of life, gave rise to
the proascus, the common primary form of the animal plants (zoophyta).
The other branch was originated by the formation of a middle germ-layer
or muscular layer, and also by the further differentiation of the
internal parts into various organs; more especially, the first formation
of a nervous system, the simplest organs of sense, the simplest organs
fo