Echinoderms are slow-moving or sessile marine animals.
Echinoderms have a coelom.
A thin epidermis covers an endoskeleton of hard calcareous plates, and most species are prickly from skeletal bumps and spines.
Unique to echinoderms is the water vascular system, a network of hydraulic canals branching into extensions called tube feet that function in locomotion and feeding.
Sexual reproduction of echinoderms usually involves separate male and female individuals that release their gametes into the water.
Echinoderms descended from bilaterally symmetrical ancestors, yet on first inspection most species seem to have a radially symmetrical form.
The internal and external parts of most adult echinoderms radiate from the center, often as five spokes.
However, echinoderm larvae have bilateral symmetry.
Furthermore, the symmetry of adult echinoderms is not truly radial.
For example, the opening (madreporite) of a sea star’s water vascular system is not central but shifted to one side.
Echinoderms do not have a respiratory, excretory, or circulatory system.
Fluids within the coelomic cavity and the water vascular system carry out many of these functions.
For example, gas exchange occurs across the skin gills and the tube feet.
Nitrogenous wastes diffuse through the coelomic fluid and the body wall.
Cilia on the peritoneum lining the coelom keep the coelomic fluid moving.
Living echinoderms are divided into five Classes.
Class Asteroidea (Sea Stars and Sea Daisies).
Sea stars have arms radiating from a central disk.
The undersurfaces of the arms bear tube feet.
By a combination of muscular and chemical actions, the tube feet can attach to or detach from a substrate.
The sea star adheres firmly to rocks or creeps along slowly as its tube feet extend, grip, release, extend, and grip again.
Although the base of the tube foot has a flattened disk that resembles a suction cup, the gripping action results from adhesive chemicals, not suction.
Sea stars also use their tube feet to grasp prey, such as clams and oysters.
The arms of the sea star embrace the closed bivalve, clinging tightly with their tube feet.
The sea star then turns part of its stomach inside out, entering it through its mouth and into the narrow opening between the halves of the bivalve’s shell.
Next, the digestive system of the sea star secretes juices that begin digesting the mollusc within its shell.
The sea star then brings its stomach back inside its body, where digestion of the mollusc’s (now liquefied) body is completed.
The ability to begin the digestive process outside of its body allows a sea star to consume bivalves and other prey species that are much larger than its mouth.
Sea stars and some other echinoderms have considerable powers of regeneration.
Sea stars can regrow lost arms, and members of one genus can even regrow an entire body from a single arm if part of the central disk remains attached.
The Class Asteroidea, to which sea stars belong, also includes a small group of armless species, the sea daisies.
Only three species of sea daisies are known, all of which live on submerged wood.
A sea daisy’s body is typically disk-shaped.
It has a five-sided organization and measures less than a centimetre in diameter.
The edge of the body is ringed with small spines.
Sea daisies absorb nutrients through a membrane that surrounds their body.
Class Ophiuroidea (Brittle Stars)
Brittle stars have a distinct central disk and long, flexible arms.
They move primarily by lashing their arms in serpentine movements.
The base of a brittle star tube foot lacks the flattened disk found in sea stars but does secrete adhesive chemicals.
Hence, like sea stars and other echinoderms, brittle stars can use their tube feet to grip substrates.
Some species are suspension feeders, others are predators or scavengers.
Echinoidea (Sea Urchins and Sand Dollars)
Sea urchins and sand dollars have no arms, but they do have five radially arranged groups of tube feet that function in slow movement.
Sea urchins also have muscles that pivot their long spines, which aid in locomotion as well as protection.
A sea urchin’s mouth, located on its underside, is ringed by highly complex, jaw-like structures that are well adapted to eating seaweed.
Sea urchins are roughly spherical, whereas sand dollars are flat disks.
Class Crinoidea (Sea Lilies and Feather Stars)
Sea lilies live attached to the substrate by a stalk.
Feather stars crawl about by using their long, flexible arms.
Both use their arms in suspension feeding.
The arms encircle the mouth, which is directed upward, away from the substrate.
Crinoidea is an ancient group whose morphology has changed little over the course of evolution; fossilized sea lilies some 500 million years old are extremely similar to present-day members of the class.
Class Holothuroidea (Sea Cucumbers)
On casual inspection, sea cucumbers do not look much like other echinoderms.
They lack spines, and their endoskeleton is much reduced.
They are also elongated in their oral-aboral axis, giving them the shape for which they are named and further disguising their relationship to sea stars and sea urchins.
Closer examination, however, reveals that sea cucumbers have five radially arranged sections of tube feet, as in other echinoderms.
Some of the tube feet around the mouth are developed as feeding tentacles.
