The New Student's Reference Work/Echinodermata

Echinodermata, the subkingdom of animals; containing starfish, sea-urchins, sea-cucumbers and crinoids. As the name implies, they all have a spiny skin. They are all inhabitants of salt water. The common starfish (see illustration) has five arms radiating from a central disk. At the end of each arm is a little red eye-spot; on the back is a rounded tubercle through which water enters the body; on the lower surface, in the center of the disk, is the mouth; and along the middle of the arms, in grooves, are the tube-feet used in locomotion. Other varieties of starfishes are the brittle stars, the basket-fish, etc. The starfishes form one class of the sub-kingdom. The sea-urchins form another class. The common sea-urchins are rounded and have a hard, limy, outer case that is covered with numerous spines (see illustration). When the spines are removed, the case is seen to be made of limy plates nicely fitted together. Five sets of these plates, passing like meridians around the shell, are perforated by little holes for the tube-feet, and five other rows of plates are not punctured. Inside the shell is a very complicated structure containing five teeth, the points of the teeth coming to the surface at the mouth; the rest of the apparatus is within the shell. It is often called Aristotle’s lantern. Other kinds of sea-urchins are flattened, and are variously called sand-cakes, sand-dollars, etc. Most of the starfishes and sea-urchins move by tube-feet, which are extended by water-pressure from within. A system of tubes, called collectively the water-vascular system, carries water over the body. The tubes are arranged as follows: a central tubular ring about the mouth and five radiating tubes leading from it into the arms. From these main tubes smaller ones spring off, right and left, communicating with the tube-feet outside the body and with an equal number of rounded sacs within the body. The water enters through a sieve-like plate on the top of the body, and is carried by a tube to the central vessel around the mouth and then distributed through the rest of the system. The sea-cucumbers form another class. They have a long, plump body shaped like a cucumber, with tube-feet in rows along the body and a tree-like expanse at one end for breathing. The crinoids or feathered stars make another class. They have a body with branching processes extending from it, all borne on a stem which sometimes is very long. They are dredged from deep-sea waters in various parts of the world. The sub-kingdom Echinodermata is represented by many fossils in the rocks. The true position of the group among animals is not clear; but some of the larvæ of Echinoderms resemble those of worms, and this serves to relate them in a general way with the worms or to indicate that the remote ancestors of both groups were similar. See Romanes: Jelly-fish, Starfish and Sea-Urchins.