Echinoderms are a diverse group of marine invertebrates, which on close inspection share similar characteristics such as a five-fold body plan, an internal skeleton of reticulate calcite plates, and a water vascular system that operates tube feet by hydraulic pressure. Many echinoderms have the ability to regrow limbs and a gut if injured by predators. Sexual reproduction is generally external, with many thousands of eggs and sperm released into the sea where fertilisation takes place, resulting in a planktonic larval stage. A few species brood their larvae.
