The non-centralized nervous system allows echinoderms to sense their environment from all sides. Different groups have different feeding habits. Members of the Crinoidea sit with arms outstretched parallel to the currents and filter feed on passing particles. Most Asteroidea are predators or scavengers, everting their stomach called a cardiac stomach , which secretes digestive enzymes on their prey. Some asteroids are also suspension feeders. Brittle stars of the Ophiuroidea are predators, deposit feeders, scavengers, and suspension feeders, which feed by outstretching their arms to capture prey.
Ophiuroids lack an intestine and anus, and therefore have an incomplete digestive system. The members of Echinoidea are suspension feeders, herbivores, detritivores, and predators.
Many have a group of hard plates which retract and grasp like teeth, commonly called Aristotle's lantern. This allows most sea urchins to graze on algae.
Most Holothuroidea are suspension or deposit feeders. Holothurians may also eviserate their digestive and other organs in response to predation or seasonal events. Echinoderms in general are most vulnerable in their larval stage. As adults, asteroids have an anti-predator adaptation where they can lose an arm to a predator and the arm is later regenerated.
Holothurians discharge sticky tubules, known as Cuvierian tubules , at a potential predator. Otters prey mainly on sea urchins. Echinoderms are usually intricate parts of their ecosystems. Many asteroids are keystone species. Sea urchins , if not controlled by predators, may overgraze their habitat. Asteroids have several commensals, including polychaetes that feed on leftovers from the sea star's prey items.
Research on echinoderms has contributed to the overall knowledge of animal fertilization and development. Many echinoderms are easy to culture and maintain in a lab setting, and produce a large amount of eggs. Sea urchin eggs are also edible and often served in sushi bars. It occurs on the coasts of Ecuador, Galapagos, Mexico and Peru.
Since they are almost exclusively marine species, echinoderms are probably osmoconformers, with little ionic regulation. It is the second largest ocean in the world after the Pacific Ocean.
Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
These ten openings are breathing and reproductive outlets, taking in water for oxygen and shedding eggs or sperm into the sea. The basket stars are a specialized type of brittle stars. They have a series of complexly branched arms which are used to catch plankton. Brittle stars are very cryptic and hide in crevices under corals. Best seen at night time, when they emerge to feed on plankton. Usually at places exposed to strong currents.
Serpent stars feed mostly on small invertebrates like mollusks, worms and crustaceans and are generally found in crevices and beneath rocks or in holes in the sand. Snake stars for example Ophiothela danae are found entwined in the branches of black corals or gorgonians where they feed on the rich mucus of their host, in turn performing cleaning functions.
As the name suggests, the arms of the brittle stars are rather liable to break. This is actually an escape mechanism. Those arms regenerate quickly and an entire new organism can regenerate, if the broken arm is attached to a seizable portion of the disk. Brittle stars can reproduce asexually by self-division. Brittle stars are the most active and fastest moving echinoderms. Radial symmetrical body with a external chitinous skeleton and a centrally located jaw called Aristotle's lantern with horny teeth.
The mouth consists of a complex arrangement of muscles and plates surrounding the circular opening. The anus is located on the upper surface.
Some sea urchins have a spherical, bulb like cloaca to store fecal material that protrudes from the anal opening. It can be withdrawn into the shell. Depending on the species, movable spines of various sizes and forms are attached to the body. These spines often are sharp, pointed and in some cases even venomous.
Pincer like pedicellaria for grabbing small prey. Some pedicellaria are also poisonous. Rubble and sand. An abundance of sea urchins can be a sign for bad water conditions. Locomotion by tube feet but also by movement of the spines on the underside of the body. Sea urchins are generally nocturnal, during the day they hide in crevices.
However some sea urchins such as Diadema sometimes form large aggregations in open exposed areas. Despite their sharp spines sea urchins are easy game for some fishes, particularly triggerfishes and puffers. A triggerfish grabs the sea urchin with its hard beak like mouth by the spines or it blows some water towards the sea urchin and turns it on its back. The underside of a sea urchin has much shorter spines and those are easily crushed. During the breeding season the body cavity is crammed with eggs or sperms.
This is one of the main reasons urchins are so attractive to fish predators Japanese also like them for the same reason. Some sea urchins are camouflaged. They hold on with their tube feet onto some bottom debris like rubble or pieces of seagrass and carry them on their back.
Some even carry live soft corals or anemones. The sexes are separate and the young are formed indirectly by the fusion of sperm and eggs released into the water. Many animals live in symbiotic relation with sea urchins. Even on the poisonous spines of the fire urchin Asthenosoma varium small shrimps Periclimenes colemani can be found.
One shrimp Stegopontonia commensalis is striped black and white lengthwise and perfectly camouflaged and lives in spines of the long-spined sea urchin Diadema setosum. Some cardinalfishes and juvenile shrimpfishes also like to take shelter in-between these spines, but even small cuttlefish hide there.
It has been observed, that they change their coloring also to black and white. Some flatworms wrap around the thicker spines of the diadema sea urchin Echinothrix calamaris. The mandarin dragonet Mandarinfish lives close to congregations of sea urchins and hides among them if threatened.
There are two specialized types of sea urchins with an unusual appearance: the sand dollar is very much flattened with very small spines and the heart urchin which are oval and have bristle like spines.
The both bury in sand. The heart urchin "jumps" out of the sand, when disturbed. Unlike other echinoderms, holothurians don't have a distinct radial symmetry but are bilateral distinct dorsal and ventral side. Holothurians are also called sea cucumbers. As their name suggests, they are cucumber shaped with an elongated, muscular, flexible body with a mouth at one end and the anus at the other.
Around the mouth there is a number of tentacles modified tube feet used in food collecting. Sea cucumbers come in many sizes, from small species only a few centimeter in length to long snakelike animals which may stretch up to 2 meter!
Most species feed on the rich organic film coating sandy surfaces. The crawl over the bottom ingesting sand. The edible particles organic matter such as plankton, foraminifera and bacteria are extracted when passing through their digestive tract and the processed sand is expelled from the anus as worm-like excrements. Sea cucumbers move by means of tube feet which extend in rows from the underside of the body.
The tentacles surrounding the mouth are actually tube feet that have been modified for feeding. Other holothurians feed on current-borne zooplankton.
They bury in sand extruding their featherlike tentacles Pseudocolochirus violaceus, Neothyondium magnum or Pentacta crassa. The tentacles have the same shape as soft corals or some anenemones. They are often mistakenly called worms. Some species resembles fat pickles a few centimeters long Fig.
Others are like thin tubes over a meter long Fig. These animals are common residents of reefs and rocky shorelines worldwide. A few species swim constantly in the water, seldom touching the bottom; they are the only members of this phylum to do so. Some Pacific islanders collect sea cucumbers, remove their intestines, and dry the muscular body wall, making a food eaten in many countries.
Unlike other groups of echinoderms, sea cucumbers have no large plates or ossicles forming a rigid skeleton. Because the spicules differ by species, they are useful in identification. Muscles in the body wall of many sea cucumbers are developed enough to aid in locomotion. When the muscles contract, the body becomes firm and rigid. In some species the muscles are so thin that the internal organs show through the body wall.
When these animals are taken from the water, the body wall collapses like thin plastic tubing. The digestive system has a mouth at one end, a digestive tube down the center, and an anus at the other end Fig. The mouth is ringed with tentacles that are modified tube feet. Some species use their tentacles to take in sediment particles rich in plant and animal matter Fig. Other sea cucumbers extend their tentacles to snatch passing food particles detritus and plankton Fig. This behavior makes them look somewhat like sea anemones, and so this class is named Holothuroidea from the Greek root word holothuroid meaning like a polyp.
The digestive tube has a stomach and a long, thin, coiled intestine where food is digested. Indigestible sand and other particles are expelled through the anus. Much the same happens in earthworms, which literally eat their way through soil. The respiratory system of sea cucumbers is unusual in its arrangement. They breathe through an internal structure called a respiratory tree , which is attached to the intestine Fig.
Seawater taken in through the anus fills this branching structure, where body fluids absorb the oxygen. Because the anus is often open during this respiratory process, other organisms—small crabs and fish among them—sometimes enter and take up residence in the lower digestive tract and respiratory tree Fig.
A few species of sea cucumbers have a set of tooth-like projections around the anus to ward off invaders. Some sea cucumbers have another bizarre way of protecting themselves. Cuverian tubules are branches of sea cucumber respiratory trees in the form of long, slender threads Fig. These Cuverian tubules contain both sticky and toxic chemicals. When these sea cucumbers are disturbed, they can eject these sticky threads out the anus, thoroughly entangling any attacking predator Fig.
The ejected tubules look like strands of limp spaghetti but stick like cobwebs. Under favorable conditions, these internal organs soon regenerate. The sea lilies and feather stars reside within the class Crinoidea from the Greek root word crino meaning lily. Sea lilies are sessile organisms attached to the substrate by a flexible stalk Figs. The digestive organs are in a bud at the top of the stalk called the calyx.
The arms of the crinoid extend out from the calyx. These arms are made up of the calcareous plates seen in other echinoderms. Like the brittle stars, they are jointed for flexibility. Each arm has am ambulacral groove containing tube feet in the center and is lined on each side with tubular extensions called pinnules. The feathery arms are used to collect food from the water, thus crinoids are filter feeders. Feather stars are similar in body form to sea lilies Fig.
Rather than an attached stalk, feather stars have small flexible appendages called cirri at the base of the calyx. These appendages allow feather stars to move around. Some feather star species can even use their arms to actively swim Fig. This document may be freely reproduced and distributed for non-profit educational purposes. Skip to main content. Search form Search. Join The Community Request new password. Main menu About this Site Table of Contents.
Home Biological Invertebrates Phylum Echinodermata. These tube feet can expand or contract based on the volume of water present in the system of that arm. By using hydrostatic pressure, the animal can either protrude or retract the tube feet.
Water enters the madreporite on the aboral side of the echinoderm. From there, it passes into the stone canal, which moves water into the ring canal. The ring canal connects the radial canals there are five in a pentaradial animal , and the radial canals move water into the ampullae, which have tube feet through which the water moves. By moving water through the unique water vascular system, the echinoderm can move and force open mollusk shells during feeding.
The nervous system in these animals is a relatively simple structure with a nerve ring at the center and five radial nerves extending outward along the arms. Structures analogous to a brain or derived from fusion of ganglia are not present in these animals. Podocytes, cells specialized for ultrafiltration of bodily fluids, are present near the center of echinoderms. These podocytes are connected by an internal system of canals to an opening called the madreporite.
Echinoderms are sexually dimorphic and release their eggs and sperm cells into water; fertilization is external. In some species, the larvae divide asexually and multiply before they reach sexual maturity. Echinoderms may also reproduce asexually, as well as regenerate body parts lost in trauma.
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