1911 Encyclopædia Britannica/Starfish

by Francis Arthur Bather

STARFISH, a popular term under which are included a large number of sea-animals, belonging all to the great group of Echinorderms, but to three distinct divisions of that
Fig 1.—An Asterid, Asterias rubens, upper surface.
a, Madreporite.
á, The same magnified.
b, Anus.
This starfish may be 9-12 in. across. group: the Asterids, the Ophiurids and the Crinoids (see Echinoderma). The Asterids or starfish proper include the cross-fish, the sun-star (see Echinoderma, fig. 17), the cushion-star, the butt-horn, and many without a popular name. The common cross-fish or five-finger, Asterias rubens, of British seas, may be taken as typical (figs. 1 and 2), and the description will apply also to the American 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 bodies—the pedicellariae (see Sea-Urchin). In the middle of the body is a small anal opening, and near the angle between two rays is a furrowed plate pierced by many minute pores and called the madreporite. The under surface of the body has the mouth in the centre, and from it deep grooves radiate to the ends of the arms. At the bottom of each groove is a water-vessel, which gives off branches to the podia or sucking-feet on each side of it. A section across this groove is given in the article Echjnoderma, fig. 12 B. The arrangement and working of this hydraulic system is essentially the same as in the sea-urchin, except
Fig. 2.—Asterias rubens, under surface.
a, The armg-groove with its row of sucking-feet or podia.
b, End of a podium, magnified. 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 Echinoderma, 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-fishers and starfish; no less than 42,000 bushels of starfish were removed from the oyster-beds of Connecticut in a single year, but not till they had worked damage to the amount of $631,500. The simplest way in which a starfish eats is by taking small bits of food into the stomach, and ejecting the refuse again through the mouth. But since the mouth is quite small and the food often large, the starfish finds it more convenient to turn its stomach inside out and to wrap it around the animal to be eaten, which is then digested quietly and the stomach withdrawn again. In the case of oysters and similar bivalves, the starfish first has to open them; and this it does by fixing the suckers of one or two rays to one valve and those of the opposite rays to the other valve, while it may get a purchase by also holding on to some neighbouring object. It then begins to straighten out its rays. The oyster can withstand a very strong pull, but it cannot hold out against a long pull, and the starfish does not hurry. At last the oyster gives way, and the starfish has its reward; but its companions often join in, and you may see a whole ball of them interlaced round half-digested molluscs and rolling about. Starfish begin to eat voraciously when quite young; one less than 3/8th in. across has been observed to eat over fifty young clams of half that length in six days. The more a starfish has to eat the quicker it grows, and it may become sexually mature in less than a year, then producing many thousands of young. Fortunately the increase is kept in check by many causes. The young, while still in the stage of free-swimming larvae, are swallowed in millions by various fish. When they settle down on seaweed their 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 single cartload thrown on shore would capture many millions. At a later stage tangles of hemp or cotton waste may, be dragged over the oyster-beds, when the starfish will cling to them by their pedicellariae. They make excellent manure, but are of ho further service to man. Fishermen who catch them in their nets or on their lines often tear them in half and throw them back into the sea. Some of these mutilated 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.

Fig. 3.—An Ophiurid, the Daisy Brittle-star (Ophiopholis aculeata); upper surface. (2/3 natural size).

The Ophiurids (the name means “snake-tails”) include the brittle-stars, 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 but through special openings (see Echinoderma, fig. 18).

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 out, so that they look like wriggling centipedes attached round a little sea-urchin. The brittle-stars are more active than the sand-stars, and can go more than two yards in a minute; some of them, if seized, break off their arms, which continue breaking into smaller pieces; but the body can soon grow new ones. Sand-stars and brittle-stars are found in all seas, usually occurring in quantities, but are most abundant in the rock-pools of the tropics. By constantly sweeping their arms over the sea bottom, they gather food consisting of minute animals. They eat the bait of fishermen, and their fish as well if they find any already dead, but they are themselves a favourite food with many fish, notably the cod.

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 of its arms, it generally fixes itself to a stone or seaweed or some zoophyte, by means of a bunch of small jointed and hooked processes (cirri) growing from the back or under side of the body. It gets its food in this way: the arm-grooves (Echinoderma, fig. 12, C) are lined with minute hairs (cilia) always waving in the direction of the mouth, towards which they drive a stream of water; this stream, containing minute organisms, constantly flows through the coiled gut, which extracts nourishment from it. The feather-stars were formerly placed with the starfish, but they really belong to another class of Echinoderms—the Crinoidea.

Fig. 4.—The Rosy Feather-star, Antedon bifida, attached by its cirri to a small stone, from which it is moving in the direction of the spectator by pushing with the branches of one arm and pulling with three branches of two arms. (Natural size.)

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. 1 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.) are found in all seas at all depths, often in enormous numbers.  (F. A. B.)