Fig. 112.—Interior of Shell or Sea Urchin
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.
Fig. 114.—Sea Urchin Dissected, showing the Digestive Tube