A number of animal phyla have developed shells mostly made out of lime, and live inside them. The most successful of these phyla is the molluscs. The gastropods, of which the snail is the best known, have what may be called a head, and typically a foot on which they crawl. Their shell is generally coiled, and they are the highest animals except the flat fish which are quite asymmetrical. Another phylum called the brachiopods makes bivalve shells which are pretty like those of the bivalved molluscs, though their soft parts are very different. Molluscs often start life as a swimming larva startingly like the larvae of some annelid worms. They are probably more nearly related to them than to the echinoderms or the round worms. The earliest echinoderms were probably fixed to the sea bottom. The best known echinoderms are starfish and sea urchins.