Echinoderms, Spiny Skins

The phylum Echinodermata ("spiny-skins") consists of about 6,000 species of exclusively marine organisms which include such well-known animals as starfish and sea urchins. The description "spiny-skins" is derived from the calcareous (chalky) plates that form the endoskeleton of these animals, which in some species is covered with spines. These spines are particularly prominant in sea urchins.

The most obvious feature of these animals is their five-rayed, or pentamerous symmetry.

Starfish

Starfish, or sea stars (class Asteroidea), are te most familiar echinoderms. They consist of a cenral disk from which five arms arise (although some species have more than 5 arms). These animals move using tube feet (podia) which are found in a groove under each arm. Each tube foot ends in a sucker which allows a starfish to stick to a rocky surface; when moving over sand, however, the tube feet are used as stiff legs.

Tube feet are also used to deal with prey which usually consists of bivalves such as clams and oysters. Within each arm there is also a pair of gonads (sex organs). Reproduction in most echinoderms is a simple process. The sexes are separate and sperm and eggs are released into the water, where fertilization occurs. The larvae that hatch are called bipinnariae and metamorphose gradually into the adult form.

In large numbers, starfish can have a devastating ecological effect as some species destroy whole areas of coral reef.

Brittle Stars
The 2,000 species of brittle stars (class Ophiuroidea), which occur at all depths of the ocean, are easily distinguished from starfish by the sharp demarcation of their central disk and by their very long arms. They also differ from starfish in several other ways. Locomotion is achieved not by tube feet but by muscular movements of the arms. Most brittle stars feed on detritus and small organisms but do not have intestine or anus, and indigestible fragments are ejected through the mouth. Starfish and sea urchins breathe by means of skin gills, which are located all over the body surface, whereas brittle stars use respiratory pouches (bursae) which occur near the arm bases.
Photo courtesy of Robert Potts © California Academy of Sciences.

Sea Urchins

Most sea uchins (class Echinoidea) have a globular body, with the skeletal plates fused together to form a hard shell (the test). In most species the body is futher protected by sharp spines which may be poisonous. The spines of some sea urchins are also used for boring holes in coral or rock into which the animals wedge themselves to prevent removal. In addition to spines, the skin of urchins and asteroids carries tiny, pincer-like structures on stalks, called pedicellariae, which are used for defense, catching small prey and cleaning the body surface. Sea urchins are omnivorous and often scavenge on organic debris. The mouth is on the underside (oral surface) and the anus on the upper side (aboral surface).

Sea Cucumbers, Sea Lilies

As their names suggests, sea cucumbers (class Holothuroidea) are sausage-shaped echinoderms, elongated along the oral-aboral axis. They have soft bodies, covered with a glandular skin, and the skeletal plates have been reduced to microscopic ossicles. Sea cucumbers burrow in sand. To breathe, they have tubes known as "respiratory trees" which carry seawater into the body from the anus, so that gaseous exchange can occur internally. The podia and skin surface are also used for respiration.
Photo courtesy of E. Eugenia Patten 1999 California Academy of Sciences

<== Photo: Sea Lilies
The lilies and feather stars (class Crinoidea) are among the most primitive of echinoderms. They resemble a starfish hat has been inverted and (in the case of sea lily) attached to the sea bed by a stalk. The sea lily skeleton which stands on a stalk, is cup-shaped with arms. Tube feet are present in a groove on the upper surface of he arms, but they are used solely for respiration. Food (usually plankton) is collected by the arms and directed to the mouth on the upper surface by cilia.

The feather stars resemble sea lilies, and, like them, develop from the larval stage into an attached form. They differ from sea lilies in that they are free-living as adults, either creeping along the sea o swimming.

Source:
1. Mike Janson and Joyce Pope (consultant editors). The Animal World
2. CalPhotos Photo Database
3. U.S. Fish and Wildlife Service Multimedia database
4. Public Health Image Library (PHIL) Photographs, Illustrations, Multimedia Files