Echinoderms are among the most distinctive of all animal phyla. They are the largest phylum to lack any freshwater or land representatives.
Echinoderms have a system of internal water-filled canals, which form tubefeet that help them grip and move around. Their skeletons are composed of calcium carbonate and proteins. Their Endoskeletons are composed of ossicles (plates of calcium) derived from single calcite crystals. They have a Water Vascular System which performs many important functions such as locomotion, respiration, and feeding. Most sensory neurons are located at the tube feet.
The adults of all echinoderms are radially symmetrical. Their size ranges from less than one centimeter to two meters. The Larvae are bilateral and range in size from about a few millimeters to a few decimeters. They have spines or spicules on their skins to a varying degree in the different groups. The larvae are attractive, planktonic creatures that are transparent and have long slender arms.
Most organs have no segmentation. The Body is covered by delicate epidermis over a firm mesodermal endoskeleton. They have no head instead the body is arranged on an oral, aboral axis.
Main organisms involved
There are about 7000 living species of echinoderms. The main types are star fish, sea urchins, sea cucumbers and sand dollars.
Starfish are usually radially symmetrical, often with 5 radiating arms joined to a central disc. The mouth is located on the underside and anus on the upper surface of the disc. Their tubefeet help in locomotion. Starfish are often formidable predators which generate frantic escape behaviors in their prey.
Sea Cucumbers are sausage shaped, and their skin is covered with warty bumps or soft spines. When they are threatened, cucumbers can contract their muscles and shoot out water from their body making them shorter, thicker, and harder. Some can even shoot out their insides and then go and grow new insides. Sea cucumbers have hundreds of tube feet to help them move around. There are three common types of sea cucumbers which are the warty sea cucumber, the California cucumber, and the white star cucumber. Warty sea cucumbers are chestnut brown with black tipped warts all over them. They grow up to ten inches long. California sea cucumbers are brown to reddish brown and are covered with pointed, cone-shaped projections. They grow up to sixteen inches long. The White Sea cucumber is light orange to white with long, non-retractable spines covering their bodies. White sea cucumbers grow up to four inches long.
Sea urchins live in the mid-level or low-level of a tide pool. They have five teeth in the middle of their back side which they use to pull, tear and rip off algae on the rocks. These teeth continue to grow throughout the sea urchin’s life. They grow to about four inches long and vary in colour but the most common are purple and light pink.
The sand dollar has five sets of pores arranged petal pattern. The pores are used to move sea water into its internal water-vascular system which allows for movement. Sand dollars live below low water on top of or under sand or muddy areas. Their flattened spines allow them to burrow under the sand. Fine hair-like cilia cover the spines. The cilia, in combination with a mucous coating, move food to the mouth opening which is in the center of the star shaped grooves on the underside of the animal. Sand dollars are often found together because of their preference to soft bottom areas as well as convenience for reproduction.
Most echinoderms do not have definite excretory and respiratory systems. Holothurians how ever have a system of tubes called respiratory trees. Water is drawn into and out of the respiratory trees, and during this process gas exchange takes place. There is usually a circulatory system present called the haemal system. It is highly developed in Holothurians and echinoids. Its channels aren’t definite vessels because they don’t have true walls and are enclosed inside coelomic channels. The digestive tract consists of a coiled tube extending from the mouth to the anus and is found on the oral surface. The reproductive system consists of a single gonad in some organisms, but usually more then one gonad is present. The gonads are arranged radially in a way that consists with the body symmetry.
Organisms of the Echinodermata phylum reproduce through both asexual and sexual reproduction. The forms of asexual reproduction that Echinoderm’s may partake in are fission and regeneration. Regeneration is the re-growth of a missing limb. Most are capable of regeneration provided that some of the central disk still remains. However certain organisms such as the asteroid Linkia, can regenerate from just an arm. Fission is the division of an organism into new organisms as a process of reproduction. Successful fission and regeneration require a body wall that can be torn and an ability to seal resultant wounds.
Sexual reproduction occurs through external fertilization. Occasionally it can occur on or in the body. There are different sexes within the organisms, and they have sex organs in each arm. Once the eggs become fertilized, they start to develop into planktonic larvae. The larvae then go through two stages; the bipinnaria and brachiolaria stages. The larvae are bilaterally symmetrical and have bands of cilia used in swimming and feeding. As the larvae gradually turn into adults, a complex reorganization and degeneration of internal organs occur. The left side of the larva becomes the oral surface of the adult which faces down. The right side becomes the aboral surface, which faces up. The larvae settle to the sea floor and result in having radial symmetry once they become adults.
Different groups have different feeding habits. Members of the Crinoids; feather stars and sea lilies, are passive feeders; meaning they absorb food particles from passing water. Crinoids sit and sieve water as it is swept by. If the arms touch any food particles present, the tube feet will sweep them into the Crinoids mouth. There it is ingested and any nutrients are removed and absorbed. Some brittle stars are passive feeders as well.
Members of Echinoidea; sea urchins, have a group of hard plates that graze rocks and scrape off the algae for food. Echinoidea who do not have these teeth will trap small organisms with their tube feet.
Sea cucumbers feed by sucking in surrounding water and through their guts and sort out any nutrients or hold out extended tentacles to trap particles and plankton. Some crawl across the ground and use tentacles to capture organic sediments.
Sea stars and star fish are mainly scavengers and carnivores. They feed by grasping slow moving prey, then everting their stomach and secreting primary enzymes on the prey. The digestive juices break down the tissue of the prey, and are absorbed. Some are suspension feeders, which trap plankton in mucus covering the body.
Basket stars, brittle stars and snake stars; the Ophiuroids, are carnivores, filter feeders, and scavengers. Some use Mucus on the tube feet to trap particles from the water, and then move them into the mouth. Some capture small crustaceans or worms.
Echinoderms are found in oceans and seas worldwide but are most commonly found in reefal environments and at off shore islands. Echinoderms are found in almost all depths, latitudes and environments in the ocean. Many crinoids live in the deep sea, but others are common on coral reefs. Some are pseudo-planktonic, attaching themselves to floating logs and debris. All six classes of echinodermata are found in New Zealand.
Star fish are benthic organisms; meaning living on the sea floor, found in marine environments, except for a few species which inhabit salty waters. Basket stars, brittle stars, snake stars and sea urchins all are marine and benthic
Echinoids are commonly grouped as regular or irregular. Sea urchins; which are regular, are generally found on rocky areas while sand dollars; irregular, are generally found on sandy or soft ground.
Sea cucumbers are common in shallow water areas as well as deep ocean floors, where they sit on the floor or burrow. A few deep-sea sea cucumbers are fully floating and can attach themselves to the sides of fish.