[Illustration: FIG. II.--The same in section. The pear-shaped cells are seen bound together in the centre of the gelatinous sphere by a thread-like process. Each cell contains both a kernel and a contractile vesicle. (PLANÆA SERIES.)--_Haeckel._] [Illustration: FIGS. III AND IV.--Represents GASTRÆA SERIES. The body consists merely of a simple primitive intestine, the wall of which is formed of two primary germ-layers.--_Haeckel._] [Illustration: FIGS. I and II.--Represents the next higher stage (Tubularia). Fig. I, a simple Gliding Worm (Rhabdocoelum); _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 gastræada 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 gastræa swam freely about in the Laurentian ocean.