Page:Popular Science Monthly Volume 5.djvu/485

Rh some failure to grow at a certain spot, while the balance of the creature continues its development, until a wall arises around the constriction and becomes permanent, further growth only increasing the introversion—precisely such a process as we may witness at the hands of a potter when he places a ball of clay upon his wheel. First he flattens the top of his ball; then, as he continues to press upon the point, the adjacent clay rises around it; and next you have a cup. And such a cup-like cell is the type of all endothentic creatures; that is, of all animals.

Naturalists, it is true, regarding only flagrant forms, confine the term, cœlenterate to one class; those that are permanently open-cupped. But, strictly, all differentiate a part of the investing tegument into a cup, for the purpose of carrying about their nutrition; and, however much the cup may be extended and contorted—drawn out into a tube and folded away into some convenient receptacle, and puckered, and tucked, and furnished with a thousand little pockets supplementary—it is all but an extension of that same original internal fold or cup. Finally, in the highest animals, all the principal vital functions are found severally the office of some pocket of the integument involuted and shoved out of sight and out of harm's way into the great cavity.

Having obtained for our animal kingdom the cupped cell as a type, we have plain sailing for some time.

In the outset, we may subdivide the kingdom into the occasionally cupped and the permanently cupped. Those that present the cup as occasion serves, or only in one stage of their evolution, are exemplified in the Rhizopoda. The permanently-cupped include not only those always open-mouthed—the cœlenterata of Mr. Huxley—but, as we have seen, necessarily all of the higher classes.

Observe, again, the India-rubber ball. First, we have the cup. Now, this open cup is mouth—is stomach—is vent—is every thing. Every thing that goes into the creature enters here; all that comes out finds exit by means of this common gap. Soon Nature, so to speak, finds this plan poor economy, and divides off one side, or edge of the cup, for one purpose, another for another purpose, and we find one corner or edge of it devoted to the entrance of nutrition, another to the exit of the débris. Nor is this mere speculation. Creatures are actually so constituted, and are seen to develop to this type from the ovum. We often see, in cœlenterates, as in sea-anemone, a tendency to the same thing. Now, this very fact of voiding indigesta at one angle of the mouth, while the other is receiving a fresh supply of food, produces a constriction in the unused middle region; and the consequence is that here the lips approximate one another, and finally, adhering at the point of contact, a permanent perinæum-like septum is formed.

This adhesion completed, nothing more is wanting to exhibit the type of the higher animalia; for here is a complete alimentary canal, however short, and a dorsal and a ventral aspect. This is an animal