Echinoderms have an endoskeleton of calcareous ossicles, hence the name Echinodermata, meaning spiny bodied. All are marine and have an organ-system level of organisation. The adult echinoderms are radially symmetrical but the larvae are bilaterally symmetrical. They are triploblastic and coelomate animals. The digestive system is complete, with the mouth on the lower (ventral) side and the anus on the upper (dorsal) side. The most distinctive feature of echinoderms is the water vascular system, which helps in locomotion, capture and transport of food, and respiration. An excretory system is absent. Sexes are separate, reproduction is sexual, and fertilisation is usually external. Development is indirect, with a free-swimming larva. Examples include Asterias (star fish), Echinus (sea urchin), Antedon (sea lily), Cucumaria (sea cucumber) and Ophiura (brittle star). The important exam points are the radial symmetry of adults versus bilateral symmetry of larvae, which is why the broad classification marks them with an asterisk, the unique water vascular system, the complete digestive system with ventral mouth and dorsal anus, and the complete absence of a separate excretory system, which is unusual among coelomates and is frequently asked as a one-line question.