<P> Starfish are marine invertebrates . They typically have a central disc and five arms, though some species have a larger number of arms . The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates . Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown . Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface . They are opportunistic feeders and are mostly predators on benthic invertebrates . Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding . They have complex life cycles and can reproduce both sexually and asexually . Most can regenerate damaged parts or lost arms and they can shed arms as a means of defence . The Asteroidea occupy several significant ecological roles . Starfish, such as the ochre sea star (Pisaster ochraceus) and the reef sea star (Stichaster australis), have become widely known as examples of the keystone species concept in ecology . The tropical crown - of - thorns starfish (Acanthaster planci) is a voracious predator of coral throughout the Indo - Pacific region, and the northern Pacific sea star is considered to be one of the world's 100 worst invasive species . </P> <P> The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death . Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate . With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture . They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten . </P> <P> The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830 . It is derived from the Greek aster, ἀστήρ (a star) and the Greek eidos, εἶδος (form, likeness, appearance). The class Asteroidea belongs to the phylum Echinodermata . As well as the starfish, the echinoderms include sea urchins, sand dollars, brittle and basket stars, sea cucumbers and crinoids . The larvae of echinoderms have bilateral symmetry, but during metamorphosis this is replaced with radial symmetry, typically pentameric . Adult echinoderms are characterized by having a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres . Starfish are included in the subphylum Asterozoa, the characteristics of which include a flattened, star - shaped body as adults consisting of a central disc and multiple radiating arms . The subphylum includes the two classes of Asteroidea, the starfish, and Ophiuroidea, the brittle stars and basket stars . Asteroids have broad - based arms with skeletal support provided by calcareous plates in the body wall while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed "vertebrae". </P> <P> The starfish are a large and diverse class with about 1,500 living species . There are seven extant orders, Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida and Velatida and two extinct ones, Calliasterellidae and Trichasteropsida . </P>

What type of symmetry does a sea star have