Echinoderms are a phylum of marine animals. The adults are recognized easily by their radial symmetry, and include such well known animals as sea stars, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssa zone. The phylum contains about 70,000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Echinoderms are also the largest phylum that has no freshwater or terrestrial representatives.
Holothuroidea
Sea cucumbers have something called pentaradial symmetry. Their posture cause them to have secondarily evolved a degree of bilateral symmetry. Most sea cucumbers have five strip-like ambulacral areas running along the length of the body from the mouth to the anus. The three on the lower surface have numerous tube feet, often with suckers, that allow the animal to crawl along. The two on the upper surface have vestigial tube feet.
In some species, the ambulacral areas can no longer be distinguished, with tube feet spread over a much wider area of the body. Apodacea have no tube feet or ambulacral areas at all, and burrow through sediment with muscular contractions of their body.
Sea cucumbers have between ten and thirty tentacles, depending on the species obviously. Many sea cucumbers have papillae. Echinoderms typically possess an internal skeleton composed of plates of calcium carbonate. Most sea cucumbers have become reduced to microscopic ossicles embedded beneath the skin. A few genera retain relatively large plates, which gives them a scaly armour.
Echinoidea
Sea urchins are bilaterans. Their early larvae have bilateral symmetry but they develop fivefold symmetry as they mature. This is most apparent in the sea urchins, which have roughly spherical bodies, with five equally-sized parts radiating out from the central axis. Several sea urchins are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet. This allows the animals to burrow through sand or other soft material.
Urchins have tube feet, which arise from the five ambulacral areas. The lower half of a sea urchin's body is referred to as the oral surface, because it contains the mouth, while the upper half is the aboral surface. The internal organs are enclosed in a hard test composed of fused plates of calcium carbonate covered by a thin dermis and epidermis. The mouth lies in the centre of the oral surface in regular urchins, or towards one end of irregular urchins. The sea urchin builds its spicules, the sharp crystalline "bones" that constitute the animal’s endoskeleton, in the larval stage. The fully formed spicule is composed of a single crystal with an unusual morphology.
The spines, long and sharp in some species, protect the urchin from predators. The spines inflict a painful wound when they penetrate human skin, but are not dangerous. It is not clear if the spines are venomous.
Urchins have tube feet, which arise from the five ambulacral areas. The lower half of a sea urchin's body is referred to as the oral surface, because it contains the mouth, while the upper half is the aboral surface. The internal organs are enclosed in a hard test composed of fused plates of calcium carbonate covered by a thin dermis and epidermis. The mouth lies in the centre of the oral surface in regular urchins, or towards one end of irregular urchins. The sea urchin builds its spicules, the sharp crystalline "bones" that constitute the animal’s endoskeleton, in the larval stage. The fully formed spicule is composed of a single crystal with an unusual morphology.
The spines, long and sharp in some species, protect the urchin from predators. The spines inflict a painful wound when they penetrate human skin, but are not dangerous. It is not clear if the spines are venomous.
Ophiuroids
The Ophiuroidea possess a skeleton of calcium carbonate in the form of calcite. In ophiuroids, the calcite ossicles are fused to form armor plates which are known collectively as the test. The plates are covered by the epidermis, which consists of a smooth syncytium. In most species, the joints between the ossicles and superficial plates allow the arm to bend to the side, but not to bend upwards. However, in the basket stars, the arms are flexible in all directions.
The ossicles are surrounded by a relatively thin ring of soft tissue, and then by four series of jointed plates. Euryalids are similar to ophiurids, if larger, but their arms are forked and branched. Ophiuroid podia generally function as sensory organs. They are not usually used for feeding, as in Asteroidea. In the Paleozoic era brittle stars had open ambicular grooves but in modern forms these are turned inward. In living ophiuroids the vertebrates are linked by well-structured longitudinal muscles. Ophiuroida move horizontally, and Euryalina move vertically. The latter have bigger vertebrae and smaller muscles. They can coil their arms around objects, holding even after death. Ophiuroida move quickly when disturbed.