Echinoderms have an endoskeleton composed of calcareous ossicles. In sea stars and brittle stars, the ossicles articulate to form flexible structures. In sea urchins and sand dollars, the ossicles are fused together to form a rigid skeletal structure known as a test.
Echinoderms exhibit a type of radial symmetry called pentamerous symmetry in which their body can be divided into five equal parts around a central axis. Echinoderms developed this symmetry as an autapomorphy. For this reason, the radial symmetry in echinoderms does not indicate that they are closely relationship to other organisms with radial symmetry such as cnidarians.
- Kindom: Animalia
- Phylum: Echinodermata
The Phylum Echinodermata contains the following subgroups:
- Class: Asteroidea (sea stars and starfishes)
- Class: Crinoidea (feather stars and sea lillies)
- Class: Echinoidea (heart urchins, sand dollars, and sea urchins)
- Class: Holothuroidea (sea cucumbers)
- Class: Ophiuroidea (basket stars, brittlestars, and snake stars)
- Class: Stelleroidea (star fishes)