Illustrated here are the larval and
adult forms of starfish class Asteroidea, phlyum Echinodermata.
These particular individuals were photographed off Lizard Island
on the Great Barrier Reef, Australia.
The Echinodermata are exclusively
marine organisms and are widely distributed upon the benthos,
from the intertidal zone to the deep sea. 'Echinodermata' means
'spiny skin', refering to their characteristically tough, spiny
exterior (see above right). The phlya comprises five extant classes:
the Crinoidea (feather
stars & sea lilies), the Asteroidea (starfish), the Echinoidea
urchins & sand dollars), the Holothuroidea (sea cucumbers)
and the Ophiuroidea (brittle stars). They number about 6000 living
species and have a fossil record extending back over 500 million
Echinoderms display a five-part radial
symmetry (see above centre), which seems to serve their sedentary
benthic lifestyle well. They possess an internal skeleton of calcium
carbonate plates, called ossicles, and a unique water vascular
system. This vascular system consists of a series of canals which
radiate throughout the body and terminate in structures called
tube feet. These tube feet penetrate the body wall and often have
a tiny suction or adhesive cup at their tip. Body fluid, with
an ionic composition similar to that of seawater, circulates through
the system for hydraulic expansion or contraction of the tube
feet. These feet have many functions, including adhesion, locomotion,
feeding and respiration.
The majority of echinoderms have
seperate sexes, but hermaphrodites occur among asteroids, holothurians
and ophiuroids. Many species have external fertilization which
produce vast numbers of planktonic larvae (see above left) that
consist of a number of distinct stages which often bear little
resemblance to the adult form.