Echinus, a genus of echinoderms, the type of the order echinoids, represented by the sea urchins or sea eggs common on our coast.

Sea Urchin, top view.

Sea Urchin, top view.

They have globular cases with flat bases, formed of calcareous plates accurately fitted together in rows of larger alternating with smaller plates, the former covered on the outside with movable spines; in some species they are 5 or 6 in. long, exceeding the diameter of the body. These spines fit by a ball-and-socket joint into little depressions, which occupy the centre of tubercles that cover the larger or ambulacral plates, and by the movement of the muscles which are attached to them they admit of considerable motion. Besides these organs of motion, upon which the weight of the animal not buoyed up by the water is sustained, hundreds of tubular feet, or ambulacra, project through openings in the smaller inter-ambulacral plates. These may be thrust out beyond the spines, and, having a little sucker at their ends, they serve to take hold of any object that comes in contact with them; and thus the animal may cause the shell to roll slowly, the spines aiding the motion. The tubular feet also serve to seize their prey, one foot after another fastening to it and passing it around to the mouth, which is in the centre of the under portion of the shell.

This being furnished with a powerful arrangement of teeth, small shellfish and crabs are easily masticated. - The echinoids are divided into regular and irregular. The former, containing the globular genera, have the mouth below and the vent above, both central, and the ambulacra in five pairs continuous from vent to mouth; the dental apparatus is complicated. They vary from 1 to 4 in. in diameter, with spines of equal length. The common sea urchin of the American coast (toxopneustes drobachiensis, Ag.) is generally about an inch in diameter, and will be easily recognized by the above general description and by the figures. Some of the large species of tropical climates are used as food. In the cidaris family, which includes those best known, are more than 20 genera. The irregular echinoids have the mouth below, the vent sometimes below, sometimes at one side, and the ambulacra not continuous. In the family clypeastridoe, in which the ambulacra resemble the petals of a flower, are the flat sea urchins, echinarachnius parma (Gray) and mellita quinquefora (Ag.), which are covered with soft velvety spines, living on sandy-shores, ' where they lightly bury themselves, only discoverable after storms or in still days when they slowly change their places.

In the globular forms the eye specks, which are at the end of the rays in the star fish, are drawn close together at the summit; at the same place is a curious little sieve or madreporic body, through which water is admitted to the five principal tubes of the interior. Sea urchins are able to bore holes in the hardest rocks, in which they lodge, enlarging the cavity as they increase in size, the opening remaining the same, so that they are prisoners for life. It is generally believed that they thus penetrate and excavate rocks by the constant motion of the microscopic vibratile cilia which cover their spines. The heart-shaped sea urchins, the spatangoids, bury themselves in mud and sand. Sea urchins are found from the upper Silurian to the present time; they were very abundant in the mesozoic age, especially in the chalk epoch.

Sea Urchin (Toxopneustes drobachiensis).

Sea Urchin (Toxopneustes drobachiensis).

Rotula.

Rotula.

Mellita quinquefora.

Mellita quinquefora.