Phylum Echinodermata Life History and Development Echinoderms are exclusively marine organisms ranging from a few millimeters to a few decimeters in size. Their larvae are microscopic, free-swimming, and bilateral becoming pentameral as adults. The majority of extant echinoderms are motile however very slow moving and only a handful can float or swim. Most reproduce externally where eggs and sperm are released into the water.
Some sea urchins brood eggs in specialized pouches.
Echinodermata encompasses a variety of lifestyles. Starfish and sea urchins are abundant in shallow water while brittle stars and sea cucumbers crave the deep sea. Sea urchins scrape the algae off rocks, starfish are predatorial, sea lilies are filter-feeders, while sea cucumbers, brittle stars, and sand dollars feed on suspended debris. Dana Krempels, 1997.