There are certain requirements of the animal organism to which every adaptation must conform. Underlying the stomachic type is the more primary fact that the natural form of colloid matter is the globe. Like all fluid or semi-fluid matter it tends to curve about a general center of attraction, in distinction to the angular extension of the crystal. This tendency shows itself in all parts of all animal forms, and also in these forms as wholes, the globe being departed from only through functional necessity, or from the superposition of a series of organs, each with a globular tendency, yielding, through mutual pressure, a more or less ovoid result.
In the Protozoa we have the globular form, diversified by temporary, improvised limbs, or by permanent organs. In the Metazoa variation from the globe takes place in axial directions—the fixed animals having usually but two axes of departure from the sphere; the moving animals having ordinarily three axes—a longitudinal, a vertical, and a transverse. The general result is the production of the round, flattened form of the two-axed, and the oval form of the three-axed animals; the further departure being in the production of limbs—appendages devoted to motion, or to assault and defense.
If now we take the Gastrula, the simple stomach-sac, for the primitive form of the many-celled animal, and the earliest phase of derivation from the Protozoa, it is easy to perceive that this hollow animal globe may vary in three different modes.
First, it may retain its sac-like form and stomach-opening, developing tentacles about the mouth, and radiated body divisions; thus passing from the single axis of the Gastrula to the double axis of the polyp.
Secondly, it may flatten, until it resembles a sack with the open top pressed down upon the bottom, and the sides bulging outward into a circle. If, now, radiated arms extend outward from this rounded side, we have the starfish type of organism.
Thirdly—still preserving its affiliation to the globe—it may lengthen instead of flattening. From this mode of development would come the longitudinal type of animals, the vermes, or worm-forms.
A still more primitive departure from the original Gastrula form is found in the sponge, in which the body-wall is pierced by minute apertures, through which food-bearing currents are drawn into the general internal stomach, and forced out again through the mouth. The low organization of the sponge results from the fact that it does not even require the mouth-arms of the polyp as an aid in food-getting. Its only motive apparatus is the cilia, or vibrating hair, of the Infusoria.
Of the three forms which thus seem to be the first natural variations of the Gastrula—the globular, the flattened, and the lengthened—the first two naturally rest on one extremity of the longitudinal or stomach axis; the other, or mouth extremity, being directed upward. Thus only these two extremities are exposed to diverse conditions, the