Echinoderms flip from bilateral larvae to five-part adults
Starfish, brittle stars, urchins, sand dollars, sea cucumbers and sea lilies make up the phylum Echinodermata, Greek for hedgehog skin. Their larvae swim with a left and right side, yet adults rebuild themselves around five parts, and they live on the seabed at every depth.
Counts of living species vary between sources, from about 7,600 to roughly 10,500, with some 13,000 extinct species known. That makes the phylum the largest found only in the sea and the second-biggest group of deuterostomes after the chordates. The first sure echinoderms appeared near the start of the Cambrian. Five classes survive: starfish, brittle stars, urchins and sand dollars, sea cucumbers, and feather stars with sea lilies. Jacob Theodor Klein coined the name in 1734 for urchins alone, and Jean Guillaume Bruguière stretched it to the whole phylum, formally in 1791.
The change of shape is dramatic. During metamorphosis the larva's left side grows while the right side is absorbed, and the survivor rearranges itself around a central axis. Five is not a fixed rule: most Leptasterias starfish have six arms, Labidiaster annulatus up to fifty, and the sea lily Comaster schlegelii two hundred. Gene studies suggest a starfish body is, in effect, almost all head.
The skeleton is made of porous calcite plates called ossicles, fused into a rigid test in urchins or jointed in arms, and sea urchins chew with a device called Aristotle's lantern. Catch connective tissue can stiffen under nervous control, letting a starfish go rigid while prying open a clam or an urchin lock its spines upright. A water vascular system of fluid canals, opening through a sieve plate called the madreporite, powers the tube feet used for moving, feeding and breathing.
Most echinoderms can reproduce asexually and regrow organs and limbs, sometimes an entire body from one arm, making them favourites of regeneration research. Their skeletons feed limestone formations, and some scientists credit their spread with driving the Mesozoic Marine Revolution.
Source: Echinoderm
