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Encyclopedia Britannica - Main :: DEM-DIO |
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DERMA , fig. 17), the cushion-star, the butt- hornf and many with- out a popular name. The common cross-fish or five- finger
rubens, of British seas, may be taken as typical (figs. 1 and 2), and the description will apply also to the American middle of the body
the madreporite. The under surface of the body
ment and working of this FIG. 2.A sterias rubens, under surface. hydraulic system is a, The arm-groove with its row of sucking- essentiallY the same as feet or podia. b, End of a podium, magnified. in the sea-urchin, except for the presence of plates at the bottom of the groove beneath the radial water-vessel, and the absence of any plates covering the groove. At the end of each ray is, as in the urchin, a single tentacle surrounded by pigment and connected with a definite plate called " terminal." Thus the terminals of a starfish correspond to the oculars of a sea-urchin (see ECII1NODERMA, fig. 3). The stomach is not a long coil, but a simple sac with branched blind tubes extending into each ray. A generative gland also passes down the side of each ray, and emits the milt or eggs when ripe through a pore near the body. Spawning takes place in spring or early summer. A starfish can crawl in any direction by means of its sucking-feet, whether the surface be hard or rough or polished, or the softest silt, whilst its supple body can squeeze through incredibly, narrow crevices. The rate of progress is about six inches a minute.The, starfish are the scavengers of the sea, but unfortunately do not confine their attentions to decaying matter; they eat oysters, clams, mussels, barnacles, sea-snails, worms, crustacea and even smaller starfish. There is constant war between oyster
oyster
bright colours attract eels and many small fishes. Later in life they are attacked by parasites, while those which stray into shallow water are eaten by gulls and crows. Freshets and cold currents are also destructive.Probably the best way in which man can keep down the numbers of starfish is by dredging the seaweed in the latter half of July when it is covered with young; a singlecartload thrown on shore would capture many. millions. At a later stage tangles of hemp or cotton
upper surface. a, Madreporite. d, The same magnified. b, Anus. This starfish may be 9-12 in. across. species A. forbesi and A. vulgaris. The animal consists of a central body or disk, produced into five arms or rays. The upper surface is covered with a' leathery skin, strengthened by a rafter-work of little bones or plates, made of crystalline carbonate of lime, many of them bearing prickles of the same substance and small pincer-like bodiesthe pedicellariae (see SEA-URCHIN). In the animals may, however, grow fresh rays, and thus one may find a starfish consisting of one large ray and four quite small ones, the whole shaped like a comet. The Ophiurids (the name means " snake-tails ") include the brittle-stabs, sand-stars, and basket-fish or medusa-heads. The two former, which may often be found hiding under the rocks, or in the seaweed, or in pools at low tide, resemble the ordinary starfish in having five distinct arms. These, however, as shown in fig. 3, are long and serpent-like, and are attached to a relatively small body or disk. The digestive and generative systems do not extend to the rays but are confined to the body. The arms are cylindrical and have no groove on the under side such as exists in starfish; but the water-vessel traverses the solid bones that form the axis of the arm, and the podia pass out through special
In Ophiurids it is the arms that are used for locomotion and not the podia, so that the latter have no terminal suckers. The axial ossicles, which correspond to the plates flooring the arm-groove in a starfish, resemble vertebrae connected by pairs of straight muscular bundles, and articulated by tenon-and-mortise joints, according to whose degree of development the arms vary in their power of coiling. These vertebrae are encased in the tough outer skin of the arm, in which are developed plates. Spines borne by these plates aid the animal in locomotion. The skin of the disk also bears small plates, which are often covered with prickles. The mouth is on the under surface of the disk, and round it are a number of short, flat processes, the mouth-papillae, which serve as strainers. Inside the mouth are seen the five tooth-plates, borne on a strong frame of complicated structure. In the sand-stars the rays are comparatively short, with their spines closely pressed to their sides, so that they look like lizards' tails; in the brittle-stars the rays are much longer and more flexible, with the spines standing
The basket-fish or medusa-heads are Ophiurids whose arms branch several times, their ends often curling and interlacing. They live in deeper water and are often brought up clinging to fishermen's lines. The feather-stars (fig. 4) have a central body and five arms, each forking at least once and fringed with small branches (pinnules) which give the feathery appearance. The mouth is in the middle of the body, and from it grooves pass along the arms and all their branches. The animal lives with the mouth upwards, and although it can crawl and even swim by movement
In 1823 J. V. Thompson, of Cork, discovered that the feather-star when quite young was fixed by a stalk, just as are nearly all crinoids (see ECHINODERMA, figs. I and 2). The stalked crinoids are not so numerous as they once were, but feather-stars belonging to about half a dozen genera (Antedon, Actinometra, &c.) ace found in all seas at all depths, often in enormous numbers. (F. A. B.) End of Article: DERMA If you wish, you can link directly to this article.
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