Echinoderms: organisms belonging to the phylum Echinodermata containing starfish, brittle-stars, sea urchins, sea lilies and sea cucumbers. They are exclusively marine, and often benthic. The body structure is characteristic, with a radial symmetry (in principle five sided) and an internal skeleton formed by calcareous plates in the skin. Usually the skeletal plates bear spines giving the echinoderms the typical spine appearance. The animals have also called water-vascular system, which is a water filled system of coelomic canals through the body, water can enter the system through a sieve-plate, called the madreporite. The system has surface appendages like the tube feet, moved by hydraulic pressure. The eggs and sperm are released in the water, and from the fertilized eggs hatch typically form larvae free living in the plankton until they settle.

Echinoids: echinoderms belonging to the class of Echinoidea, like the heart urchins, sea urchins and sand dollars.

Alternative form for echinoderms : echinoids.