Echinoderms are generally benthic marine organisms, meaning that they tend to live on the ocean bottom, often (though not always) in relatively shallow near-shore environments. You'll note that many of their features are adaptive for this "high energy" environment in which they may be constantly buffeted by waves and may even be exposed at low tide. Most species reproduce sexually, have separate sexes, and have external fertilization. Note that the larvae of echinoderms are ciliated and bilaterally symmetric (shown at top right).

Many echinoderms are recognized by their pentaradial (five-fold) symmetry. It is important to understand that the radial symmetry of echinoderms is superficial and almost certainly secondarily derived. This means that the ancestor of echinoderms (the ancestor it shares in common with all other bilaterians) was likely bilaterally symmetric. If we look carefully at a typical echinoderm like the sea star (see diagram below right) we will note that the five part organization can be divided into mirror-image right and left halves - the hallmark of bilateral symmetry. An easy way to see this is to find the madreporite and imagine a line connecting it to the tip of the arm opposite the madreporite. This line will divide the animal into right and left halves.

Three other shared derived features distinguish the echinoderms as a group.

1. An endoskeleton composed of calcium carbonate plates called ossicles. Although it may appear otherwise, the fact that the ossicles are covered by a layer of epidermis defines the skeleton as an endoskeleton. Also note that the skeletal material of echinoderms, calcium carbonate (CaCO₃, is distinct from that found in the endoskeletons of vertebrates. Vertebrate endoskeletons are made of calcium phosphate.
   
   
2. The water vascular system, which is a system of hydrostatic organs involved in locomotion, respiration, and feeding. This system is blind ended, with the madreporite being the valve controlling the movement of sea water into and out of the system. The madreporite communicates with a centrally located ring canal via a single stone canal, named for the calcification of its walls. The ring canal, as the name suggests, encircles the central disc and gives off radial canals into each of the animal's arms. These radial canals give rise to many short, valved lateral canals that end in tube feet. Tube feet are muscular and consist of an ampulla (which acts as a pump, pressurizing the tube foot when the lateral canal valve is closed) and a podium (which acts as a foot, contacting surfaces in the environment). The various canals of the water vascular system are ciliated and always fluid-filled.
   
   Note that adhesion by the tube feet is primarily chemical - the tube foot secretes a substance that bonds with the surface and another chemical that breaks those bonds and releases the tube foot. This type of adhesion makes sense considering that echinoderms may need to grip very tightly to surfaces for long periods of time to resist wave action. If it depended on muscular contraction long-term, strong adhesion that would require a great deal of energy.
   
   Because the tube feet are very thin-walled, their surface is suitable for the diffusion of oxygen into the body cavity and the diffusion outward of carbon dioxide and wastes. Many groups have developed auxiliary respiratory structures, for example, sea stars typically have short gills extending up between the ossicles of their aboral surfaces.
   
   
3. Special collagenous tissues which are normally rigid, but can be temporarily unlocked, allowing echinoderms to maintain postures without muscular effort - again, an efficient solution to the problem of holding on for long periods of time in a high energy environment. For more information on these "mutable connective tissues" - Natural Histroy Magazine: Catch and release - sea cucumbers might put a torn Achilles tendon back together again.

Larval echinoderm. Note obvious bilateral symmetry. For more information and images of larval echinoderms, see Marine Invertebrate Larvae: a study in morphological diversity at the University of Saskatchewan Archives.

 

Anatomy of a sea star. The surface of a sea star is covered by spines that help defend against predators and by small gills for gas exchange. Internal organs are suspended by mesenteries in a well-developed coelom. A short digestive tract runs from the mouth on the bottom of the central disk to the anus on the top of the disk. Digestive glands secrete digestive juices and aid in the absorption and storage of nutrients. The central disk has a nerve ring and nerve cords radiating from the ring into the arms. The water vascular system consists of a ring canal in the central disk and five radial canals, each running the length of an arm in a groove. The system connects to the outside by way of the sievelike madreporite. Branching from each radial canal are hundreds of hollow, muscular tube feet filled with fluid continuous with the rest of the water vascular system. Each tube foot consists of a bulblike ampulla and suckered podium (foot portion). The podium expands and extends to contact the substratum when the ampulla squeezes water into it. The podium shortens and bends when muscles in its wall contract, forcing water back into the ampulla. (From Campbell 6th edition's Fig 33-38.)

A typical sea star(From Brusca and Brusca, 2003). Note that a line connecting the tip of “arm A” and the madreporite will pass through the anus and will define a plane of bilateral symmetry for the animal.

Examples:

The phylum Echinodermata includes five morphologically distinct classes:

• Asteroidea (sea stars)
• Ophiuroidea (brittle stars)
• Echinoidea (sea urchins)
• Holothuroidea (sea cucumbers)
• Crinoidea (sea lilies)

Examples of each class are shown at right. From top:  The test, or skeleton, of a sea urchin is globular and made up of closely interlocking calcareous plates.  Here the test is surrounded by dried sand dollars, which are like flattened sea urchins. (photo: Ali Pivoda) 

Sand stars (Astropecten foliota) can be recognized by the rows of spines along the margins of their arms and can attain a diameter of up to 10 inches. They range from the Channel Islands off California to Ecuador and eat a variety of foods including snails, dead fish, sand dollars, clams, other sea stars, and sea urchins.(photo: Channel Islands NMS - courtesy of NOAA National Marine Sanctuaries)

The basket star is a type of brittle star with branching arms. (photo:  Ali Pivoda)

Here a sea otter feeds on a sea urchin in Kachemak Bay National Estuarine Research Reserve (photo courtesy of NOAA National Estuarine Reserve Collection)

Class Asteroidea