STARFISH, any echinoderm of the group Asteroidea (see ECHINODERMATA), in which a pentagonal disc forms the central body, whence radiate five or more pointed arms, containing prolongations of the stomach, as well as gestive glands, reproductive organs, etc. The mouth is on the inferior surface of the disc and the vent (when one exists) is on the upper surface. The exoskeleton consists of a mosaic of limy plates united and covered by a leathery, usually reddish-brown skirt, forming a pliant armor. The plates of the disc are similar to those of the "cup') of crinoids (q.v.) and similar echinoderms. Those of the arms form along the floor a groove carrying the lacral water-vessels and tube-feet. The exterior of the plates, except in the ambulacral grooves, is studded with tubercles, each bearing a spine, giving the a generally spiny appearance. Studding the skin among the spines arc numer cus branchial papilla: and the usually three jawed, flexible-stalked pedicellariie, which con tinually bend about and by their snapping pick up and remove accumulated waste matter from the respiratory surface of the animal. So inde pendently do they act that for many years they were considered to be parasites. The alimen tary canal is simple, with a pair of large =cal digestive glands reaching into each arm. (The nervous, blood and water-vascular systems con form to the plan described in the article ECH INODERMATA). Distinct eye-spots are present at the tips of the rays in most species, the large orange-colored spot which is popularly called the "eye° of the starfish being in reality the madreporic plate, by means of which water is admitted to the ambulacral system. In starfishes the sexes are usually separated and the eggs fertilized in the water. Development takes place with a metamorphosis, the larval stage be ing free-swimming, provided with ciliated bands and processes, and known as the bipinnaria. The body of the young starfish forms within the body of the larva, only a small portion of which develops directly into the starfish, the remainder being simply absorbed to nourish it.
Locomotion is accomplished by means of the thousands of tube-feet arranged in double rows on the under side of each arm. These succes
sively extend, attach themselves, contract, let go and reach forward again, hundreds acting to gether to draw the body slowly but continuously onward. The individual feet are extended by means of water forced into them by the con traction of the ampulla, with which each is pro vided, and contracted by their own musculature, which forces the water back into the now re laxed ampulla, the supply of water .in any part being regulated through the madreporic plate and the system of canals and polian vesicles. Besides the sexual mode of reproduction star fishes possess the power of regenerating lost parts developed to so great a degree that it amounts to a sexual reproduction. When han dled or injured they will often automatically cast off an arm or even all of the arms close to the base. Not only will the disc regenerate any number or all of its arms when lost, but in some cases the single arms will develop new discs, thus producing several individuals from one. Experimental cuttings of starfishes has shown how bifurcated or extra arms may be produced. As their only natural enemies are bottom-feeding fishes of the cod family, and as it seems probable that one or more arms could thus often escape capture, this capacity for regeneration (q.v.), combined with their tenacity of life and enormous fecundity, prob ably accounts for the surpassing numbers in which they sometimes occur.
All starfishes are marine and most of them live in comparatively shallow water, though a few extend to great and many to moderate depths. As fossils they occur in most forma tions from the Cambrian upward, but are much less numerous than the sea-urchins, being less well adapted by their structure for preserva tion. While the number of arms is generally five, or some multiple of five, some genera bear irregular numbers as 13 in Solaster, 9 to 13 in Brisingo, or upward of 20 in Hellaster, while a few, as Culcita and Asteriscus, are pentag onal and nearly without trace of arms.