Also called Coelenterata [se-LEN-ter-AH-tah], these develop from a diploblastic (two-layered) embryo, and have two separate tissue layers and radial body symmetry. Many of these have two life stages, the mobile, usually bell-like medusa and the sessile polyp. All of these have nematocysts, or stinging cells, for capturing prey, and some can inflict painful stings on swimmers. Examples include the hydras, sea anemones, corals, jellyfishes, and Portuguese man-o-war. The most diverse and successful animal phylum on earth (incorporating about 75% of all described animal species), these are characterized by jointed legs and a chitinous exoskeleton. Like annelids, they are segmented, but unlike annelids, their segments are usually fused into larger body parts with specialized functions (such as the head, thorax, and abdomen of an insect). These are often divided into four subphyla: Uniramia (insects, centipedes, millipedes); Chelicerata (arachnids, sea spiders, horseshoe crabs); Crustacea (shrimps, lobsters, crabs, crayfish, barnacles, pillbugs), and Trilobitomorpha (the trilobites, now extinct). These are second in diversity only to the arthropods. Body plans within this phylum are diverse, but general characteristics include a soft body covered by a thin mantle, with a muscular foot and an internal visceral mass. There are two fluid-filled body cavities derived from mesodermal tissue; a small coelom and a large hemocoel that functions as an open circulatory system. Many of these have a shell composed of calcium carbonate and proteins, secreted by the mantle. Familiar groups within this phylum include the classes Gastropoda (slugs, snails), Bivalvia (clams, oysters, scallops), and Cephalopoda (nautilus, squids, octopi).