Echinoderms are radially symmetrical animals that are only found in the sea (there are none on land or in fresh water). Echinoderms mean "spiny skin" in Greek. Many, but not all, echinoderms have spiny skin. There are over 6,000 species. Echinoderms usually have five appendages (arms or rays), but there are some exceptions.
Radial symmetry means that the body is a hub, like a bicycle wheel, and tentacles are spokes coming out of it (think of a starfish). As larvae, echinoderms are bilaterally symmetrical. As they mature, they become radially symmetrical.
Most adult echinoderms live on the bottom of the ocean floor. Many echinoderms have suckers on the ends of their feet that are used to capture and hold prey, and to hold onto rocks in a swift current.
The differences between the two sub-types lies in how the arms connect to the central disk. Ophiuroids have arms that do not connect with each other. There is a distinct boundary between arm and central disk. Asteroids have arms that are connected to each other. Also, it is harder to tell with asteroids where the central disk ends and the arms begin.
The sea star's top surface (or skin) looks spiny if you examine it. If you look very closely you will notice that there are different types of growths on the surface. Some bumps are used to absorb oxygen, they are called dermal branchiae. Pedicellaria are pincher-like organs used to clean the surface of the skin. Barnacle larvae could land on a sea star and start growing if it were not for these organs.
How Do Sea Stars Move?
Each ray of a sea star has a light sensitive organ called an eyespot. Though it can not see nearly as well as we do, sea stars can detect light and its general direction. They have some idea of where they are going.
Can Sea Stars Grow New Arms?
What Do Sea Stars Eat?
Sea Urchins & Sand Dollars
The spines are connected to the skeleton in a ball-joint manner called the "test." Spines are able to swivel towards a predator because they are connected to muscles.
These animals have five paired rows of tube feet with suckers. The mouth is on the bottom, and the anus on top. The mouth contains five teeth that point towards the center. The tubed feet are used to move along the bottom of the sea, where these organisms use their teeth to eat algae.
Sea cucumbers belong to a sub-category of echinoderms called holothuroidea. Some members of the group look like cucumbers you find in the supermarket. The similarity ends there.
Sea cucumbers are football shaped creatures that lay on their side at the bottom of the ocean. They have five rows of tube feet running lengthwise. Its mouth is surrounded by tentacles that are really tubed feet. Unlike sea stars, the vascular system is not filled with sea water. Instead, sea cucumbers use a special body fluid.
Sea cucumbers eat plankton and other organic matter. Some position themselves in a current that brings a steady supply of food, and spend hours there. The tentacles open up and collect food in the current. The sea cucumber brings each tentacle to its mouth to eat, while the other tentacles go on collecting food. Other sea cucumbers feed by sifting through sand using their tentacles.
How Do Sea Cucumbers Protect
The sea cucumber has an interesting way of defending itself--a sea cucumber can expel (throw out) all of its internal organs! This either scares off or satisfies predators. The sea cucumber can then grow another set of internal organs.
Some sea cucumbers secrete a very sticky substance as a defense mechanism. If you get this glue on your body, you will not be able to remove it without shaving your skin!