ECHINUS 387 certain of the rows of plates are perforations for the ambulacra or suckers used in locomo- tion, and between them the spine-bearing plates, when these exist. They have well de- veloped muscles, and a nervous ring around the O3sophagus, which sends off branches to the rays, which are generally five or a multiple of five. Respiration is by means of branchiae, by organs performing other functions, and by water passing into the general cavity of the body. They reproduce by means of eggs, and abound in almost all seas ; some of the lower orders were among the earliest created ani- mals, and the class ranges from the Silurian to the present epoch. They embrace the five or- ders of crinoids or stone lilies, ophiurans or brittle stars, asterioids or star fishes, echinoids or sea urchins, and holothurians or sea cucum- bers, the last being the highest, and seemingly a connecting link between radiates and articu- lates. There, is a remarkable persistency of form in the class from the earliest to the pres- ent time, some of the oldest being referred by Goldfuss to existing genera. They are essen- tially carnivorous. ECHINUS, a genus of echinoderms, the type of the order eehinoids, represented by the sea urchins or sea eggs common on our coast. 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 out- side 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 ad- mit of considerable motion. Besides these or- gans of motion, upon which the weight of the animal not buoyed up by the water is sus- tained, 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 fur- nished with a powerful arrangement of teeth, small shellfish and crabs are easily masticated. The echinoids are divided into regular and ir- regular. The former, containing the globular Sea Urchin (Toxopneustes drobachiensis). genera, have the mouth below and the vent above, both central, and the ambulacra in five pairs continuous from vent to mouth ; the den- tal apparatus is complicated. They vary from 1 to 4 in. in diameter, with spines of equal length. The common sea urchin of the Ameri- can coast (toxopneustes drolacTiiensis, Ag.) is generally about an inch in diameter, and will be easily recognized by the above general de- scription and by the figures. Some of the Eotula. 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 clypeastridce, in which the ambu- lacra resemble the petals of a flower, are the flat sea urchins, echinarachnius parma (Gray) and mellita quinquefora (Ag.), which are cov-
