Echinoderms are a phylum composed of marine animals, including (but not limited to) starfish, crinoids, sea urchins, sand dollars, and sea cucumbers. Their numbers are approximately 7,000 species strong, and they are found the world over in almost every marine environment known. Echinoderms breed by either releasing gametes into the ocean in hopes that they reach a mate, or by reproducing asexually. The larvae are free-swimming, and will eventually metamorphose from being bilaterally symmetrical into a radially symmetrical form, generally consisting of five-points. As an adult, an Echinoderm is known by these characteristics: a water vascular system (allowing gas movement and nutritional distribution), external tube feet for locomotion, as well as a calcareous endoskeleton that is comprised of numerous plates called ossicles. Echinoderms feed by a variety of means. Some are grazers like most sea urchins, while some are filter-feeders like crinoids, and others are hunters like most starfish. Their methods of feeding can also keep their ecosystems thriving, whether it’s by reducing algae, releasing nutrients by boring into rocks, or introducing oxygen to even smaller organisms by digging into the earth. The oldest positively identified Echinoderm fossils known come from the Early Cambrian period of approximately 540 million years ago, though certain older fossils are disputed by experts as being members of the phylum. With their ossified skeletons, Echinoderm fossils have contributed immensely to limestone formations, and help provide an idea of what the marine environments were like for their times in the prehistoric past.
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