Page:Popular Science Monthly Volume 27.djvu/378

 five grooves, each extending to the tip of one of the five rays (Fig. 7). On each side of these grooves are many actively moving membranous tubes, which are used for crawling, and are called the pedicels or feet. They are closed at the free end, but communicate by a system of tubes within the body of the animal with the madreporic tubercle. It has been surmised that this tubercle acts as a filter to the sea-water which, with some admixture, forms the liquid circulating in the tubes, and Dr. Romanes has proved the surmise to be correct; for colored fluid, injected under pressure into any part of the system of tubes, found its way to the madreporic tubercle, and oozed through its porous substance. The tube-feet are thrust forth or withdrawn by being distended with liquid or emptied. With the exception of a few at the tip of each arm, every tube-foot bears a sucker (Fig. 4); these suckers are pressed closely to a flat surface by filling the tube-feet with liquid; the pressure within the tubes is then lessened, and the greater pressure of the surrounding water holds the suckers fast. They are released by increasing the pressure of the liquid within the tube-feet.

The common star-fish usually crawls in a determinate direction, the feet on the tip of the foremost ray being used as feelers. In a tank, when the star-fish has ascended the side and reaches the surface of the water, it often performs peculiar movements which may be called acrobatic. The animal does not wish to leave its native element—in fact, can not do so, because its sucking feet can act only under water—neither does it wish to descend at once.



It therefore crawls along the side of the tank, now and then throwing back its uppermost ray or rays to feel about for rocks or sea-weed (Fig. 5). If it finds any solid support it will very likely attach its