Page:First course in biology (IA firstcourseinbio00bailrich).pdf/276

 , an urchin with mouth (o) and vent (A) on same side of body.

The largest species of sea urchins are almost as big as a child's head, but this size is unusual. The spines are mounted on knobs, and the joint resembles a ball-and-socket joint, and allow as wide range of movement. Some sea urchins live on sandy shores, other species live upon the rocks. The sand dollars are lighter colored. (Why?) They are usually flatter and have lighter, thinner walls, for there is danger of sinking into the sand. The five-holed sand cake or sand dollar has its weight still further diminished by the holes, which also allow it to rise more easily through the water. The flattened lower surface of both starfish and sea urchin causes the body to remain still while the tube feet are stretching forward for another step.

The sea cucumbers, or holothurians, resemble the sea urchin in many respects, but their bodies are elongated, and the limy plates are absent or very minute. The mouth is surrounded by tentacles (Fig. 65).

The brittle stars resemble the starfish in form, but their rays are very slender, more distinct from the disk, and the tube feet are on the edges of the rays, not under them (Fig. 66).