Introduction of Echinoderms

The echinoderms have always been one of the sea's unique groups. When man first classified the echinoderms, they were placed with the coelenterata because of their radial symmetry. They were given a name, the Radiata, based on this one characteristics. As our biological knowledge increased through research, the differences between the two groups became obvious. In fact, about the only thing these two groups share is radial symmetry, and in echinoderms this is an adult trait; larval symmetry is bilateral. Their radial symmetry separates both groups from the great majority of the rest of the animal world, but they differ from each other because the echinoderms have a complete digestive system with a mouth, intestine, and anus. They also have a water vascular system, found only in echinoderms. The general characteristics of the phylum, then, are a water vascular system, solely marine distribution, and a skeleton composed of calcareous plates that lie under their skin. Their radial symmetry, when present, gives them the advantage of encountering their environment equally as well from any direction. Almost all forms are benthic, so their substrate protects them from attack from below and their spiny skins, spines, or leather-like hide, depending on their class within the phylum, protect them from attack on the upper side (Reseck, 1979).

The echinoderms are the only major group of animals that are all confined to the marine world. Among the over 5000 species of echinoderms, sestars are the most familiar and often are the first animals that come to mind when we describe life in the sea. Most are abundant on all rocky shores throughout the world (Lerman, 1986).

Echinoderms are remarkable animals that include the sea lilies, feather stars, brittle stars, sea stars, sand dollars, sea urchin, sea biscuits, and sea cucumbers. Recent analysesindicate that echinoderms together with hemichordates from the sister group to chordates. Nearly all of the approximately 6500 echinoderm species living today are marine; a few species are estuarine, but none live in feshwater. An additional 13.000 or so species, distributed among approximately 16 classes, are known from the fossil record; most of those classes have no living representatives. Although their free-living larval stages are bilaterally symmetrical, most adult echinoderms show a basic 5-point (pentamerous) radial symmetry. As radially symmetrical animals, they lack cephalization. Thus, adult echinoderms generally do not have anterior and posterior ends. Instead, body surfaces are designated as being either oral (bearing the mouth) or aboral (not bearing the mouth). Most echinoderms possess a well-developed internal skeleton composed largely (up to 95%) of calcium carbonate, with smaller amounts of magnesium carbonate (up to 15%), even lesser amounts of other salts and trace metals, and a small amount of organic material. The components of the echinoderms skeleton are individually manufactured within specialized cells originating from embryonic mesoderm. This is in sharp contrast to the method of shell production in molluscs and other invertebrate groups, in which minerals are deposited into an extracellular protein matrix (Pechenik, 2005).

One of the most distinctive characteristics of echinoderms is the unusual way they move. The organ system associated with locomotion and feeding is called water vascular system, a unique system that uses water -filled tubes and water pressure. Some water enters the system of tubes leading to hundreds of tube feet through the sieve plate or madreporite. It appears that the ampulla associated with each tube foot is the basic or functional structure in the water vascular system. The ampullae fill with water, then contract and exert pressure on the tube feet. The contaction of the ampulla (which is similar to squeezing a medicine dropper) allows the tube feet to elongate or extend.  The expanded tube foot then is moved by muscles. Some echinoderms heve little suction cups at the tip of each foot, with exert a small pull when the muscles in the tube foot contract after the foot touches something solid. When many tube feet attach to an object, the coordinated action of the feet exerts a powerful force. In this way, seastars are able to pull apart the shells of a clam or oyster. Among the echinoderms, there is great variability in the structure and function of the tube feet. However, most spiny-skinned animals creep slowly over the bottom by movements of the tube feet and associated muscles (Lerman, 1986).

tube feet on Acodontaster sp

 The echinoderms are generally divided into four classes. Stelleroidea is the class and asteroidea the subclass that we normally call sea stars or starfish. The subclass Ophiuroidea are sea stars with dishlike body and very slender arms, generally called brittle or serpent stars. Crinoidea, unlike the other two star types, have mouth turned up and are called, among many other common names, sea lilies and feather stars. Two classes are not sea stars of any sort and to the casual observer, very little resemble the members of the two classes already mentioned. The sea urchins and sand dollars belong to the class called Echinoidea and have a shell (or test, as it is called in sea urchin). The other class has a tough skin and no shell. The sea cucumber belong to this class, called Holothuroidea. Representatives of the phylum and, indeed, of most of the classes are found around the world in most seas and at most depths (Reseck, 1979)

Literature:

Lerman, Matthew. 1986. Marine Biology. The Benjamin/Cummings Publish. California, 534pp.

Pechenik, Jan. 2005. Biology of the Invertebrates fifth edition. Mcgraw Hill International Edition. New York.

Reseck, John. 1979. Marine Biology. Reston. Virginia, 257pp.