One of the most interesting features of this urchin is undoubtedly its complex and wonderful masticating system. There are five teeth, symmetrically arranged, and all pointing towards the centre of the mouth. Each is attached to a wedge-shape jaw, made up of several pieces, and the whole apparatus is attached by ligaments to loops in the interior of the shell, and is moved by no less than thirty distinct muscles. The complete system may be readily dissected out, and is well worthy of study and preservation. (The harder portions of the system may often be found in the interior of the empty shell of an urchin after the softer structures of the body have decayed away.) [Illustration: FIG. 112.--INTERIOR OF SHELL OR SEA URCHIN] [Illustration: FIG. 113.--MASTICATORY APPARATUS OF SEA URCHIN] An interesting dissection of the globular urchin may also be made by cutting completely round the shell with a pair of sharp-pointed scissors midway between the mouth and the apex, and then separating the upper and lower halves, as shown in fig. 114. In this way the whole of the digestive tube, with its numerous curves, may be traced from the mouth to the anus at the opposite pole. The water-vessels that supply the tube-feet in the regions of the five perforated plates may also be seen, as well as the ovaries or egg-producing organs and the bases of the five jaws with their complicated system of muscles. A little acquaintance with the commonest of the British sea urchins will show that they may be divided into two well-defined groups, one containing the globular or subglobular forms, of which the common sea urchin or sea egg (_Echinus sphæra_) above described, is a type, as well as the pretty little Green Pea Urchin (_Echinocyamus pusillus_), and the little Purple-tipped Urchin (_Echinus miliaris_), which is found principally on the west coast of Scotland; while the second group is formed by the less symmetrical Heart Urchins, which differ from the others in several interesting particulars of structure and habit. [Illustration: FIG. 114.--SEA URCHIN DISSECTED, SHOWING THE DIGESTIVE TUBE] These heart urchins (Plate IV., fig. 4) are covered with short, delicate spines which are not much used for purposes of locomotion, the animals moving from place to place almost entirely by means of their tube-feet, while the globular urchins travel principally by their spines, which are stouter and more freely moved on well-formed ball-and-socket joints. Also, while in the globular species the perforated plates that admit of the protrusion of the feet are arranged with a perfect radiate symmetry, those of the heart urchins are confined to one side of the shell; and the digestive tube, which in the former terminates in the pole opposite the mouth, in the latter ends close to the mouth itself. Further, the heart urchins do not possess any kind of dental apparatus. [Illustration: PLATE IV ECHINODERMS