Usually, for sea stars and most other members of this group, the symmetry is 5-fold. The group is Phylum Echinodermata, which means 'spiny-skinned'. Despite their adult radial symmetry, echinoderms give away their ancestral symmetry in their larval stages. A sea star goes through several stages with bilateral symmetry, including the bipinnaria stage shown on the Marine Larvae Gallery Page. From these observations and other evidence, biologists have concluded that echinoderms probably evolved from an ancestor with bilateral symmetry - perhaps a worm allied to the bryozoans or their ancestors.
Most echinoderms spawn directly into the sea.
The fertilized eggs develop through several microscopic swimming stages, and in manny species these larvae feed on smaller plankton for several weeks before the animal is ready to settle to the bottom. About our video program on Phylum Echinodermata. This material may be copied for use in student classroom projects and reports. Limited permission is provided for teachers and educational web sites. No other use of this material is allowed without written permission.