Page:Popular Science Monthly Volume 27.djvu/377

Rh and sea-urchins, or more particularly to the nervous systems, and the movements controlled by them, in these creatures.

A star-fish, as we all know, consists of a central disk and five radiating arms (Fig. 1). Upon the whole of the upper surface occur numerous calcareous nodules imbedded in the soft flesh, and supporting short spines. One of these nodules, much larger than the others, is always found a little to one side of the center, and is called the madreporic tubercle (Fig. 1, m). With the aid of a lens we may see also on the upper surface a number of small organs, each consisting of a pair of pincers supported on a flexible stalk, scattered about among the calcareous nodules, or attached to the spines; these are known as pedicellaræ (Fig. 2). These organs are provided with muscles' by which the stalk is swayed about, and the pincers are opened and shut. What it is that these curious organs are adapted to seize, and therefore of what use they are in the economy of the animal, has long been a puzzle to naturalists, but Dr. Romanes and his associate, Professor Ewart, have succeeded in throwing some light on this point. In some species of star-fish the size of the central disk is increased so as to fill up the spaces between the rays, the form of the animal thus becoming a pentagon. In other species the reverse process has taken place, the rays having become relatively longer, and, being at the same



time very active, they look like five little snakes joined together by a small circular disk (Fig. 3). Again, in another species the rays branch, these branches again branch, and so on till the animal looks like a mat. Turning now the under surface of our star-fish uppermost, we see that the mouth is in the center of the disk, and that from the mouth radiate