Phylum Echinodermata

What is an echinoderm? PHYLUM ECHINODERMATA Members of this phylum have many unusual characteristics. They move by means of hundreds of appendages that have suction cups on them. Their skin is covered by tiny, jaw-like pincers. They are also found in all the oceans of the world.

A. Echinoderms have endoskeletons All echinoderms have a hard, spiny, or bumpy endoskeleton covered by a thin epidermis. Sea urchins have long, pointed spines. Sea stars (sometimes called starfish) have long, tapered arms called rays. These rays are covered with short, rounded spines. A sea cucumber has soft tissue that has small, plate-like structures in it. These are the spines. The endoskeleton of all echinoderms is made mostly of calcium carbonate, the compound that makes up limestone.

Echinoderms

Some of the spines found on sea stars and sea urchins have modified into pincer-like appendages called pedicellariae. These are used for protection and for cleaning the surface of the body.

B. Echinoderms have radial symmetry Radial symmetry is an advantage to animals that are stationary or that move slowly. This type of symmetry enables these animals to sense potential food, predators, and other aspects of their environment from all directions.

C. The water vascular system The water vascular system enables echinoderms to move, exchange gases, capture food, and excrete wastes. This is a hydraulic system that operates under water pressure. Water enters and leaves the system through the madreporite, a disk-shaped opening that is like a filter. It is located on the upper surface of the echinoderm’s body. This disk functions like the strainer that fits into a sink drain and keeps large particles out of the pipes.

Echinoderms On the underside of a sea star, tube feet run along a groove on the underside of each ray. Tube feet are hollow, thin-walled tubes that end in a suction cup. They look like miniature droppers. The round, muscular structure called the ampulla works like the bulb of a dropper. Each tube foot works independently of the others, and the animals moves by alternately pushing out and pulling in its tube feet.

Echinoderms Tube feet also function in gas exchange and excretion.

D. Echinoderms have varied nutrition All echinoderms have a mouth, stomach, and intestines, but their methods of obtaining food varies. Sea stars are carnivores and prey on worms or mollusks such as clams. Most sea urchins are herbivores and graze on algae. Brittle stars, sea lilies, and sea cucumbers feed on dead and decaying mater that drifts down to the ocean floor.

E. Echinoderms have a simple nervous system Echinoderms have no head or brain, but they do have a central nerve ring that surrounds the mouth. Nerves extend from the nerve ring down each ray. Each radial nerve then branches into a nerve net that provides sensory information to the animal. Echinoderms have cells that detect light and touch, but most do not have sensory organs. Sea stars are an exception. A sea star’s body is made up of long, tapering rays that extend from the animal’s central disk. A sensory organ called the eyespot is made up of light- detecting cells and is located at the tip of each arm, on the underside. Eyespots enable sea stars to detect the intensity of light.

Echinoderms Most sea stars move toward light. Sea stars also have chemical receptors on their feet. When it detects a chemical signal from a prey animal, it moves in the direction of the arm receiving the strongest signal.

F. Echinoderms have bilaterally symmetrical larvae Echinoderms in the larval stage have bilateral symmetry. Through metamorphosis, the free-swimming larvae change their body parts and symmetry.

Diversity of Echinoderms There are about 6000 species of echinoderm that exist today. Class Asteroidea sea stars Class Ophiuroidea brittle stars Class Echinoidea sea urchins and sand dollars Class Holothuroidea sea cucumbers Class Crinoidea sea lilies and feather stars Class Concentricycloidea sea daisies

A. Sea Stars These are probably the most familiar echinoderm. Most species of sea star have 5 rays, but some have more. Some may have more than 40 rays.

B. Brittle Stars Brittle stars are extremely fragile. If you try and pick it up, parts of its rays will break off in your hand. This adaptation helps the brittle star survive attacks from predators. While the predator is busy with the broken off ray, the brittle star can escape, and a new ray will regenerate. Brittle stars do not use their tube feet for movement. They move like a snake, using their flexible rays. They use tube feet to pass food along the rays and into the mouth in the central disk.

C. Sea Urchins and Sand Dollars These are globe-shaped animals covered with spines. They do not have rays. The circular, flat skeletons or a sand dollar have a 5-petal flower pattern on the surface. A living sand dollar is covered with tiny, hair-like spines that are lost when the animal dies. A sand dollar has tube feet that come out of the petal markings on its upper surface. These tube feet are modified into gills and are used for respiration. Tube feet on the animal’s bottom surface help bring food to the mouth.

Sea Urchins Sea urchins look like living pincushions and have long, pointed spines. These spines protect it from predators. Sea urchins have long, slender tube feet that help with movement.

D. Sea Cucumbers Sea cucumbers get their name because they look like a vegetable. They have a leathery covering that allows them to be flexible as they move along the ocean floor. When they are threatened, they may give off a tangled sticky mass of tubes through their anus, or they may simply rupture, releasing some internal organs that will be regenerated in a few weeks. These actions confuse their predators, giving the sea cucumber a chance to move away. They reproduce by shedding eggs and sperm into the water, where fertilization occurs.

E. Sea Lilies…. These organisms resemble plants in some ways. Sea lilies are the only sessile echinoderms.

…and Feather Stars Feather stars are sessile only in the larval stage. The adult feather star uses its feathery arms to swim from place to place.

F. Sea Daisies Two species of sea daisies were discovered in 1986 in the deep waters off of New Zealand. They are flat, disk-shaped animals less than 1 cm in diameter. Their tube feet are located around the edge of the disk rather than along radial lines.