Page:Popular Science Monthly Volume 17.djvu/625

Rh of digested and undigested matter. This perfect stomach is realized so gradually, or by such slight degrees, in a large number of lower animals, that it is not easy to say positively which animal has the honor of its first possession. To the little sea-urchin, which is the first possessor of true teeth, is generally given the credit. The ctenophore, a lower animal, has indeed two or more apertures to its food-cavity, but the mouth is still the main excretory orifice, and this cavity freely communicates with a system of body canals. The starfish, on the contrary, has the stomach distinct from the other cavities of the body, with, however, in most cases only a single opening.

In the sea-urchin we also find an intestine and indications of the several parts which are so distinct in higher animals. The sea-cucumber,

 (after Owen): o, gullet; c, crop; p, proventriculus; g, gizzard; sm, small intestine; k, intestinal cæca; l, large intestine; cl, cloaca.

a near relative of the former but with a better digestive canal, is almost as highly favored regarding its stomach as the low amoeba. If its stomach becomes troublesome from indigestion or other cause, it simply ejects it through the mouth, along with its other internal organs. Then it quietly awaits the growth of a new set—certainly a very happy and efficient method. Many human dyspeptics would rejoice in the same power. This animal is said to reject its viscera when it is injured or alarmed. This is interesting, as showing in the low animals that which is well known in the highest, the immediate effect of fear and pain upon the internal organs, or the close dependence of the nutritive organs upon the nervous system.

Among articulates and mollusks we find a great diversity in the character of the digestive canal. Its main divisions are always shown