Page:Popular Science Monthly Volume 71.djvu/476

470 the picture upon the screen. There is a little creature, which many of you have seen in the garden, consisting of joints, which rolls itself up into a little ball, and therefore is often called the "pill-bug." It is not, however, an insect or a bug, properly so-called, but belongs to a family of crustaceans. It has on its head a little feeler which we call the antenna. The particular kind of arthropod, the antenna

of which has been studied and drawings of it made to furnish us this plate, is known by the name of Oniscus. In his researches the experimenter. Dr. Ost, cut off the antenna in the middle of a joint and found that it rapidly healed over. Here is pictured the process of healing going on. Part of the antenna has been cut off in this case, the wound was healed over here, No. 1, a, the new tissue has begun to grow, No. 2, b, and the cells at this point are very simple in character. They spread out and grow, and then, within the interior of the hard shell of the feeler, a retraction of the substance occurs, and the new growing cells within this space gradually begin to shape themselves out.