Page:Popular Science Monthly Volume 17.djvu/623

Rh of all the other vegetative functions. A single opening serves both to receive the food and expel the waste matter. Within even this narrow limit of structure, we find a host of low animals which exhibit a great variety of forms; so that from the hydra to the ctenophore is a progressive series showing a gradual specialization of this common



organ. In the higher part of the series, as for example the sea-anemone, there is a digestive cavity somewhat separated from the body cavity, though still connecting; and all the excretions have to find their way out through the oral aperture.

In the compound hydrozoans, produced by budding and division, such as sertularia and the so-called corals, the body cavity is continuous through the whole community. Hence each individual (though

 Actinia holsatica (after Frey and Leuckart): a, mouth; b, gastric cavity; c, common cavity, into which the gastric cavity and the intermesenteric chambers open; d, intermesenteric chambers; e, thickened free margin, containing thread-cells of, f, a mesentery; g, reproductive organ; h, tentacle.

it is scarcely correct to regard it as such) has its stomach connected with the stomachs of all the others. Whatever food one digests serves to nourish the whole colony. They are absolute communists.

At this point should be presented the fact that in all animals the lining or secreting membrane of the food-canal is essentially but a continuation of the skin. That such is true of the amœba is evident, for what was the outside of the body-mass becomes when food is enveloped the lining of the new cavity. The cup-shaped body of the hydra can