A class within phylum Echinodermata, the name Asteroidea comes from the Greek aster (star) and oeides (in the form of). Composed of sea stars and sea daisies.

Characteristics of Sea Stars

  • Pentaradial symmetry with five arms that radiate from a central disc.
  • An aboral or topmost surface covered in spines.
  • Calcium Carbonate plates called ossicles that form the skeleton.
  • Tube feet, suction discs that allow the sea star to move and affix itself to the substrate.
  • Dermal Branchiae, creases between ossicles that function in gas exchange.
  • A nervous system that allows for calculated suction of the tube feet.
  • Dioecious; males and females externally indistinguishable.
  • Larval form is bilaterally symmetrical, looks like a pair of melted goggles.
  • Feed on clams, snails, crustaceans, coral, and polychaetes.
  • Able to project a portion of its stomach into bivalve shells to release digestive enzymes.
  • Able to regenerate an entire star from one arm and a portion of the central disc.

Characteristics of Sea Daisies

At one time, the sea daisies belonged to their own class, Concentricycloidea, but recent evidence points to the sea daisy being a specialized sea star. The sea daisy does not have arms, instead being composed of two vascular rings surrounding the central disc. The outer ring has tube feet attached to it. Sea daisies are less than 1 cm in diameter and have no internal digestive system, but use a membrane know as velum to absorb nutrients from decomposing organisms.


Animal Diversity Web: Asteroidea
Campbell & Reece. "Biology" Benjamin Cummings. 6th Edition, 674
Miller & Harley. "Zoology" McGraw Hill. 7th Edition, 266-269