[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.
1. Asterias rubens 2. Goniaster equestris 3. Ophiothrix fragilis 4. Echinocardium cordatum 5. Echinus miliaris 6. Echinus esculentus]
The habits of sea urchins are interesting, and may be watched in the aquarium, where the movements of the spines and of the tube-feet may be seen perfectly. Some species are very inactive, living in holes and crevices, or under stones, and seldom move from their hiding-places, while others travel considerable distances. The former have generally no eyes, and, instead of seeking their food, simply depend for their subsistence on the material carried to them by the movements of the water; while the latter possess visual organs similar to those observed in certain starfishes. Some species also protect themselves from their enemies when in the open by covering their bodies with sand, small stones, shells, or weeds, and thus so perfectly imitate their surroundings that they are not easily detected. The feet that are used for purposes of locomotion terminate in suckers resembling those of the common five-fingered starfish, and have considerable clinging power, but some have either very imperfectly developed suckers or none at all, and are probably used as feelers only.
Sea urchins, like their allies the starfishes, generally inhabit deep water beyond low-water mark, where they often exist in enormous numbers, feeding on both animal and vegetable substances; but some species are often to be met with between the tide-marks, where they may be seen under stones, and frequently half hidden in mud. The globular species occur principally on rocky coasts, but the heart urchins are more commonly dredged from banks of sand or mud that are always submerged.