Fig. 52.—Sea urchin without its spines.
Fig. 53.—Short-spined sea urchin, showing the biting teeth.
Some sea urchins have short spines (Fig. 53) and are almost pure white; some are flat like the sand dollars, the spines feeling like sandpaper, so short and fine are they. The latter are small, and appear to be covered with waving filaments. Many have spines like needles; in others the latter are blunt, clublike organs. Many other strange variations are seen in an exhibition of the various kinds in some museums. None are more remarkable than those having five holes through them like Chinese money (Fig. 54).
Fig. 54.—Flat sea urchin, "sand dollar."
The urchins are very closely allied to the starfishes, especially in structure. They have the same kind of feet, and among the spines is seen a singular handlike organ common to the starfishes. It has three fingers (Fig. 55) and a short stem, and is constantly in motion, its office appearing to be to clean the body. Foreign objects are taken up by this peculiar hand and passed on from one to the other until they are finally dropped off. Here is the same madreporic plate or sieve, and the structure of the Echinus (Fig. 56) is very similar to that of the starfish. The former has a long set of jaws, hence is a biter and nipper, while the starfish is a sucker. The shell of the Echinus is really a beautiful object when divested of the spines and bleached in the sun, appearing as pure white as coral after bleaching. It is made up of about six hundred hard, limy plates arranged in double rows, which contain about thirty-seven hundred pores through which the feet protrude. Despite this marvelous supply of feet, or organs of locomotion, the Echinus is a very slow walker. The spines number four thousand or more, and each one works on the ball-and-socket plan, is hollow, and moves readily in all directions.
