Echinoderm

An echinoderm is a spiny-skinned invertebrate (an animal without a backbone) that lives in the ocean. Most echinoderms, like starfish and sand dollars, have a distinctive five-part body plan, an endoskeleton (an internal skeleton), and many tiny, sucker-tipped tube feet with which they take in seawater in order to move.

As one of the more unusual animals, the echinoderm (meaning "spinyskinned") numbers about 6,000 related species and includes such exotic animals as the five-armed starfish, the pincushion-like sea urchin, and the bottle-shaped sea squirt. With their armor-like external spines that protect them, it might seem that an echinoderm has an exoskeleton (like a lobster or an insect). In fact, the spines are only extensions of an interior or endoskeleton that makes echinoderms unique among invertebrates. This internal skeleton is made of plates under the skin that have spiny projections. Because of this internal skeleton, echinoderms are considered closer to vertebrates that any other invertebrate phylum. However, unlike vertebrates, echinoderms have no head or centralized nervous system. Nor do they have an excretory or respiratory system. Finally, echinoderms have a circulatory system that is unique in the animal kingdom since they use seawater as their circulatory fluid. Echinoderms use their specialized tube feet to suck in water and create a suction effect that allows them to move about and to feed on other animals.

The starfish is an excellent example of features that make an echinoderm such a different animal. Also called sea stars, this invertebrate has a radial symmetry, meaning that its body parts are arranged around a central area or hub. Animals with radial symmetry can be divided into mirror-image halves along many lengthwise lines. A human body that has bilateral symmetry only can be divided perfectly down the middle and, therefore, have only two mirror-image halves. However, a starfish with five arms can be divided evenly several different ways. A starfish has a skeleton made up of hard plates beneath its skin. It also has many canals inside its body that help it operate a water pumping system, allowing it to move about and find the food it needs. These canals are connected to tube feet that are located on its underside and are hollow, suction-cup-like structures that suck in water. This forces the tube feet outward. A starfish can walk on the tips of its arms, using the sucker action of the tube feet to cling to a surface. When the suction is released, it moves forward. The starfish also uses its tube feet to wrap its arms around a clam shell and pry its hinged shells apart. A starfish's mouth is located in the center of its underside. It eats the soft-bodied mollusk by pushing part of its own stomach through its mouth and engulfing its prey, thereby digesting the clam inside the shell.

Finally, the starfish and all other echinoderms reproduce sexually by the union of male sperm and female egg. They also have remarkable powers of regeneration. If an echinoderm's arm breaks off, it will grow back, and if a part of its central core breaks off, it will grow into a complete organism. Many clam and oyster fishermen viewed the starfish as a rival, since clams and oysters are the starfish's main diet, and would tear these creatures apart whenever they found them. Unfortunately the fishermen were only multiplying their problem.

[See alsoAnimals; Invertebrates ]