noderms evolved from animals with bilateral symmetry. Although adult echinoderms possess pentaradial, or five-sided, symmetry, echinoderm larvae are ciliated, free-swimming organisms that organize in bilateral symmetry which makes them look like embryonic chordates. Later, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis. Within the Asterozoa, there can be a few exceptions from the rule. The starfish genus Leptasterias normally have six arms, although five-armed individuals can occur. Also the Brisingida have six armed species. Amongst the brittle stars, six-armed species such as Ophiothela danae, Ophiactis savignyi, and Ophionotus hexactis exists, and Ophiacantha vivipara often has more than six. Echinoderms exhibit secondary radial symmetry in portions of their bo dy at some stage of life, an adaptation to their sessile existence. They developed from other members of the Bilateria and have bilaterally symmetric larvae. Many crinoids and some seastars have symmetry in multiples of the basic five, with starfish such as Labidiaster annulatus known to possess up to fifty arms, and the sea-lily Comaster schlegelii having two hundred. Skin and skeleton A brittle star, Ophionereis reticulata A sea cucumber from Malaysia Starfish exhibit a wide range of colours Strongylocentrotus purpuratus, a well-armoured sea urchin Crinoid on a coral reef Echinoderms have a mesodermal skeleton composed of calcareous plates or ossicles. Each one of these, even the articulating spine of a sea urchin, is composed mineralogically of a crystal of calcite. If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereom. Ossicles may be fused together, as in the test of sea urchins, or may articulate with each other as in the arm s of sea stars, brittle stars and crinoids. The ossicles may be flat plates or bear external projections in the form of spines, granules or warts and they are supported by a tough epidermis (skin). Skeletal elements are also deployed in some specialized ways, such as the "Aristotle's lantern" mouthparts of sea urchins used for grinding, the supportive stalks of crino
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