(10,000 species) Also called Coelenterata [se-LEN-ter-AH-tah], the phylum develop from a diploblastic (two-layered) embryo, and have two separate tissue layers and radial body symmetry. Many animals from this phylum have two life stages, the mobile, usually bell-like medusa and the sessile polyp. All animals 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 (which is actually an aggregation of colonial variant of this phylum). (over 800,000 species described; estimates of actual diversity vary but go as high as 9 million species) The most diverse and successful animal phylum on earth (incorporating about 75% of all described animal species), the animals from this phylum 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). The phylum is 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). (50,000 species) This phylum is the 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 animals have a shell composed of calcium carbonate and proteins, secreted by the mantle. Familiar groups within the phylum include the classes Gastropoda (slugs, snails), Bivalvia (clams, oysters, scallops), and Cephalopoda (nautilus, squids, octopi). (6,500 species) Characteristics of this phylum include an endoskeleton composed of many ossicles of calcium and magnesium carbonate, a water vascular system (WVS), a ring canal around the esophagus, and locomotion by tube feet connected to the WVS. Unique to the phylum is the five-fold radial symmetry obvious in sea stars (seafish), sea urchins, and sea lilies. Others, like sea cucumbers, have varying degrees of bilateral symmetry. In the body plan, a true head is absent; the anatomical terms oral (mouth-bearing) and aboral (away from the mouth) are used to describe orientation of the body surfaces. Feeding adaptations include particle feeding through the WVS, everting the stomach to engulf prey (sea stars), and a scraping device called Aristotle's lantern (sea urchins).