Internal and external parts radiate from the center, often as five spokes. A thin skin covers a hard calcareous plate-like exoskeleton. Most have bumps and spines which serve various functions. A unique feature of Echinoderms is the water vascular system . A network of hydraulic canals which branch into extensions called tube feet that function for locomotion, feeding and gas exchange.
Echinoderms are dioecious with sexual reproduction and external fertilization.
Bilaterally symmetrical larvae metamorphose into radial adults. Early embryonic development exhibits the characteristics of deuterostomes. There are about 7,000 species of echinoderms, all of which are marine. This class includes the sea stars which have five or more arms extending from a central disc. Tube feet on the undersurface of the arms are extended by fluid forced into them by contraction of their ampulla .