Almost the sole function of the spines seems to be that of protecting the animal, and enabling it to resist the attacks of its enemies, the force of the waves, or any sudden violent contact with the rocks. The spines, when magnified, are seen to be finely ribbed for nearly the whole length (Fig. 135), the bare basal knob serving as the point of attachment for the powerful muscles, which move these spines on a regular ball-and-socket joint, the ball surmounting the tubercles (seen in Fig. 132), which fit exactly in a socket at the base of the spine. In a transverse section of a spine (Fig. 136), we see that the ribs visible on the outside are delicate columns placed closely side by side, and connected by transverse rods forming an exceedingly delicate pattern. Beside the tentacles and the spines, they have other external appendages, of which the function long remained a mystery, and is yet but partially explained; these are the so-called pedicellariæ; they consist of a stem (_s_, Fig. 137), which becomes swollen (_p_, Fig. 137) into a thimble-shaped knob at the end (_t_, Fig. 137); this knob may seem solid and compact at first sight, but it is split into three wedges, which can be opened and shut at will. When open, these pedicellariæ may best be compared to a three-pronged fork, except that the prongs are arranged concentrically instead of on one plane, and, when closed, they fit into one another as neatly as the pieces of a puzzle. [Illustration: Fig. 136. Transverse section of spine; magnified.] [Illustration: Fig. 137. Pedicellaria of Sea-urchin; _s_ stem, _p_ base of fork, _t_ fork.]