The fertilized egg has two forms: the blastula and the gastrula. These swim close to the surface of the water with the aid of cilia, and can be dispersed quite far, depending on currents. These larvae are known as the echinopluteus, and can remain in the larval stage for an average of 4-6 weeks. As the larvae mature, a vestibule is created in what will be the oral side of the urchin. Tentacles grow from this opening, on which suction areas eventually emerge. When the tentacles have suckers, they are primary poda, which serve as locomotive tools when the larva sinks to the ocean floor. At this point the skeletal plates begin to develop. When the 5 ambulical plates are developed and the terminal plate lies next to the genital plates, the urchin is fully developed, though it will continue to grow for the rest of its life.
Development - Life Cycle: metamorphosis
- Grzimek, B. 1972. Grzimek's Animal Life Encyclopedia. New York: Van Norstand Reinhold Company.