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ing itself and changing its place if necessary. At the end of the animal opposite to the fixed base is placed the mouth, in the centre of a rounded, smooth space, which is surrounded by a fringe of numerous hollow processes or "tentacles."
Both the body and tentacles are formed out of two membranes, an outer and an inner (shown by the dark and light lines in fig. 5, B); and the outer layer of the skin contains a vast number of microscopic stinging organs or "nettle-cells," essentially similar to those which we have already seen in the Fresh-water Polype. The skin, also, is very contractile, and the animal can pull itself together into a mass when irritated, or even when left uncovered by the retreating tide.
The tentacles are hollow, and communicate with the cavity of the body. They can be drawn in at will, and are the organs by which the animal captures its prey. At their bases is seen a circle of from eight to twentyfive bright blue spots, which are perhaps rudimentary eyes.
The internal anatomy of the Sea-anemone is of a very simple character. The mouth opens into a globular stomach (fig. 5, B, b), and this opens below by a wide aperture into the general cavity or space included within the walls of the body. This last-mentioned space is filled with sea-water, mixed with the products of digestion, and it is subdivided by a number of upright membranous plates, which wall off the body-cavity into a series of chambers or compartments.
The digested portions of the food pass through the stomach into the general cavity of the body, and the indigestible portions are got rid of through the mouth. There is no nervous system, nor are there any breathingorgans, nor any organs of the circulation.
The Actinia mesembryanthemum is strictly marine, and is found abundantly on the coasts of Britain between tide-marks, adhering to stones, or expanding its beautiful flower-like disc in shallow rock-pools. It varies extremely in colour, being usually of a liver-brown or olive-green colour, but not uncommonly being purplish-red, or grassgreen; and it attains a diameter of an inch or an inch and a half.
Recapitulation Of Essential Characters. — The body is made up of two distinct layers, enclosing a "body - cavity" which communicates freely with the outer world through the mouth. There intervenes, however, between the mouth and the body-cavity a globular stomach. The integument is furnished with "nettle-cells." There is no nervous system (with few exceptions), and distinct organs of respiration and circulation are not developed. These characters distinguish the class Aetinozoa as a whole.
The commonest of the animals which are included in this class are generally known as Sea-urchins, Star-fishes, Brittle-stars, Sand-stars, and Sea-lilies. Most of these common names refer to the fact that the body in these animals is generally more or less star-like in shape. The name "Sea-urchin" and the technical name "Echinodermata," on the other hand, refer to the fact that the skin in these animals is generally rendered prickly, like that of a hedgehog, with numerous spines, tubercles, and grains of lime (Greek, echinos, a hedgehog; derma, skin). As an example of this class we may take the common Star-fish or Cross-fish (Uraster rubens) of British seas.
The most conspicuous point about the form of the Starfish is its strikingly star-like shape (fig. 6). It consists, namely, of a by no means well-marked central body or disc, from which spring five (sometimes four or six) blunt finger-like processes or "arms." The arms, in fact, form a star, and the body looks as if it were composed of the
bases of the arms united together. The body of the Starfish is therefore said to have a "radiate" structure (Latin, radius, a ray).
The entire upper surface of the body in the Star-fish is covered with a leathery skin, from which project numerous blunt conical spines or prickles composed of lime. Along the middle of the back of each arm, these spines form a well-marked zigzag line, more conspicuous in some examples than in others. Between the spines also are much smaller, stalked prickles, which can be seen when magnified to terminate in little pincers.
In the centre of the under surface of the body is placed
the opening of the mouth, surrounded by a fringe of long spines. From the mouth radiate five broad and shallow grooves, which proceed along the under surface of the five arms, gradually tapering towards their extremities. Each of these grooves is bordered by a row of long spines, and each has its floor formed by a double row of little plates (fig. 7, a a), which run across the groove, and are so shaped as to leave a little opening between every pair of plates in the series.
It follows from the above, that if we cut the arm of the Star-fish across, we find that it is rounded above, and
Fig. 7.—One of the arms of the Star-fish, cut across, to show the groove on the under surface. a a Transverse plates which form the floor of the groove; 6 The water-vessel, with the little " feet" proceeding from it; n Nerve-cord.
hollowed out or grooved below (fig. 7). If we examine the animal in its living state, we find each groove under the arms to be occupied with four rows of little delicate membranous tubes, which end in little suckers. These tubes are called the "feet," because it is by the combined action of these that the Star-fish creeps about. The "feet" can be thrust out to a great length, and they spring from a common tube which runs along the bottom of the groove below the arm. (They are shown in fig. 7 at b, but here only two of the rows of feet are shown, the other two being omitted for the sake of clearness). The tubes which run along the grooves under the arms, as well as the "feet" proceeding from them, may be called "water-vessels," because they are filled with water from without. The sea-water, in fact, is admitted to them by means of a little grooved, rounded tubercle which is seen on the back of the animal (fig. 6) placed at the angle where two of the arms unite.
The mouth of the Star-fish is not provided with teeth, and opens into a very thin and membranous stomach, which can be thrust forth or "pouted out" through the mouth. From the stomach proceed ten much-branched membranous sacs, two of which are prolonged into each of the arms. The stomach terminates in an intestine, which opens by a minute vent upon the back of the animal.
The nervous system has the form of a circular cord surrounding the mouth, and sending a branch along the groove in each of the arms. At the tip of each arm there is also seen a small reddish spot, surrounded by a circle of spines, and these are of the nature of rudimentary eyes.
There are no distinct breathing-organs, and the process of respiration appears to be chiefly carried on by the absorption of the sea-water through the skin of the back, the delicate membrane which lines the interior of the animal being protruded for this purpose in the form of small tubes which project through interspaces in the integument.
The common Star-fish usually measures from three to six inches across, and is generally of a reddish, yellowish, or orange colour. During life its skin, though very rough and prickly, is comparatively soft; but it contains so much lime that it can be excellently preserved simply by drying it in the sun. It is entirely a native of the sea, and is found from low water to depths of twenty or thirty fathoms. It is very voracious, and feeds upon oysters and other shell-fish, seeming to suck the animal out of the shell by means of the protrusible stomach. Lastly, the Star-fish has the power of reproducing its arms when broken off or injured.
Recapitulation Of Essential Characters —The body exhibits more or less clearly a star-like arrangement