Echinoderm
An echinoderm is any animal of the phylum Echinodermata, which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, as adults echinoderms are recognisable by their usually five-pointed radial symmetry, and are found on the sea bed at every ocean depth from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after the chordates, as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian.
Echinoderms are important both ecologically and geologically. Ecologically, there are few other groupings so abundant in the deep sea, as well as shallower oceans. Most echinoderms are able to reproduce asexually and regenerate tissue, organs and limbs; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified dermal endoskeletons, which are major contributors to many limestone formations and can provide valuable clues as to the geological environment. They were the most used species in regenerative research in the 19th and 20th centuries. Further, some scientists hold that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution.
Etymology
The name echinoderm is.The name Echinodermata was originated by Jacob Theodor Klein in 1734, but only in reference to echinoids. It was expanded to the phylum level by Jean Guillaume Bruguière, first informally in 1789 and then in formal Latin in 1791. In 1955, Libbie Hyman attributed the name to "Bruguière, 1791 ."
This attribution has become common and is listed by the Integrated Taxonomic Information System, although some workers believe that the ITIS rules should result in attributing "Klein, 1778" due to a 2nd edition of his work published by Leske in that year.
While Echinodermata has been in common use since the mid-1800s, several other names had been proposed. Notably, Francis Arthur Bather called the phylum "Echinoderma" in his 1900 treatise on the phylum, but this name now refers to a fungus.
Diversity
There are about 7,600 extant species of echinoderm as well as about 13,000 known extinct species. All echinoderms are marine, but they are found in habitats ranging from shallow intertidal areas to abyssal depths. Five extant classes of echinoderms are generally recognized: the Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, and Crinoidea.Anatomy and physiology
Echinoderms evolved from animals with bilateral symmetry. Although adult echinoderms possess pentaradial symmetry, their larvae are ciliated, free-swimming organisms with bilateral symmetry. Later, during metamorphosis, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis. Within the Asterozoa, there are a few exceptions from the rule. Most starfish in the genus Leptasterias have six arms, although five-armed individuals can occur. The Brisingida also contain some six-armed species. Amongst the brittle stars, six-armed species such as Ophiothela danae, Ophiactis savignyi, and Ophionotus hexactis exist, and Ophiacantha vivipara often has more than six.Echinoderms have secondary radial symmetry in portions of their body at some stage of life, most likely an adaptation to a sessile or slow-moving existence. Many crinoids and some seastars are symmetrical in multiples of the basic five; starfish such as Labidiaster annulatus possess up to fifty arms, while the sea-lily Comaster schlegelii has two hundred.
Genetic studies have shown that genes directing anterior-most development are expressed along ambulacra in the center of starfish rays, with the next-most-anterior genes expressed in the surrounding fringe of tube feet. Genes related to the beginning of the trunk are expressed at the ray margins, but trunk genes are only expressed in interior tissue rather than on the body surface. This means that a starfish body can more-or-less be considered to consist only of a head.
Skin and skeleton
Echinoderms have a mesodermal skeleton in the dermis, composed of calcite-based plates known as ossicles. If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereom. Ossicles may be fused together, as in the test of sea urchins, or may articulate to form flexible joints as in the arms of sea stars, brittle stars and crinoids. The ossicles may bear external projections in the form of spines, granules or warts and they are supported by a tough epidermis. Skeletal elements are sometimes deployed in specialized ways, such as the chewing organ called "Aristotle's lantern" in sea urchins, the supportive stalks of crinoids, and the structural "lime ring" of sea cucumbers.Although individual ossicles are robust and fossilize readily, complete skeletons of starfish, brittle stars and crinoids are rare in the fossil record. On the other hand, sea urchins are often well preserved in chalk beds or limestone. During fossilization, the cavities in the stereom are filled in with calcite that is continuous with the surrounding rock. On fracturing such rock, paleontologists can observe distinctive cleavage patterns and sometimes even the intricate internal and external structure of the test.
The epidermis contains pigment cells that provide the often vivid colours of echinoderms, which include deep red, stripes of black and white, and intense purple. These cells may be light-sensitive, causing many echinoderms to change appearance completely as night falls. The reaction can happen quickly: the sea urchin Centrostephanus longispinus changes colour in just fifty minutes when exposed to light.
One characteristic of most echinoderms is a special kind of tissue known as catch connective tissue. This collagen-based material can change its mechanical properties under nervous control rather than by muscular means. This tissue enables a starfish to go from moving flexibly around the seabed to becoming rigid while prying open a bivalve mollusc or preventing itself from being extracted from a crevice. Similarly, sea urchins can lock their normally mobile spines upright as a defensive mechanism when attacked.
The water vascular system
Echinoderms possess a unique water vascular system, a network of fluid-filled canals modified from the coelom that function in gas exchange, feeding, sensory reception and locomotion. This system varies between different classes of echinoderm but typically opens to the exterior through a sieve-like madreporite on the aboral surface of the animal. The madreporite is linked to a slender duct, the stone canal, which extends to a ring canal that encircles the mouth or oesophagus. The ring canal branches into a set of radial canals, which in asteroids extend along the arms, and in echinoids adjoin the test in the ambulacral areas. Short lateral canals branch off the radial canals, each one ending in an ampulla. Part of the ampulla can protrude through a pore to the exterior, forming a podium or tube foot. The water vascular system assists with the distribution of nutrients throughout the animal's body; it is most visible in the tube feet which can be extended or contracted by the redistribution of fluid between the foot and the internal ampulla.The organisation of the water vascular system is somewhat different in ophiuroids, where the madreporite may be on the oral surface and the podia lack suckers. In holothuroids, the system is reduced, often with few tube feet other than the specialised feeding tentacles, and the madreporite opens on to the coelom. Some holothuroids like the Apodida lack tube feet and canals along the body; others have longitudinal canals. The arrangement in crinoids is similar to that in asteroids, but the tube feet lack suckers and are used in a back-and-forth wafting motion to pass food particles captured by the arms towards the central mouth. In the asteroids, the same motion is employed to move the animal across the ground.
Other organs
Echinoderms possess a simple digestive system which varies according to the animal's diet. Starfish are mostly carnivorous and have a mouth, oesophagus, two-part stomach, intestine and rectum, with the anus located in the centre of the aboral body surface. With a few exceptions, the members of the order Paxillosida do not possess an anus. In many species of starfish, the large cardiac stomach can be everted to digest food outside the body. Some other species are able to ingest whole food items such as molluscs. Brittle stars, which have varying diets, have a blind gut with no intestine or anus; they expel food waste through their mouth. Sea urchins are herbivores and use their specialised mouthparts to graze, tear and chew their food, mainly algae. They have an oesophagus, a large stomach and a rectum with the anus at the apex of the test. Sea cucumbers are mostly detritivores, sorting through the sediment with modified tube feet around their mouth, the buccal tentacles. Sand and mud accompanies their food through their simple gut, which has a long coiled intestine and a large cloaca. Crinoids are suspension feeders, passively catching plankton which drift into their outstretched arms. Boluses of mucus-trapped food are passed to the mouth, which is linked to the anus by a loop consisting of a short oesophagus and longer intestine.The coelomic cavities of echinoderms are complex. Aside from the water vascular system, echinoderms have a haemal coelom, a perivisceral coelom, a gonadal coelom and often also a perihaemal coelom. During development, echinoderm coelom is divided into the metacoel, mesocoel and protocoel. The water vascular system, haemal system and perihaemal system form the tubular coelomic system. Echinoderms are unusual in having both a coelomic circulatory system and a haemal circulatory system, as most groups of animals have just one of the two.
Haemal and perihaemal systems are derived from the original coelom, forming an open and reduced circulatory system. This usually consists of a central ring and five radial vessels. There is no true heart, and the blood often lacks any respiratory pigment. Gaseous exchange occurs via dermal branchiae or papulae in starfish, genital bursae in brittle stars, peristomial gills in sea urchins and cloacal trees in sea cucumbers. Exchange of gases also takes place through the tube feet. Echinoderms lack specialized excretory organs and so nitrogenous waste, chiefly in the form of ammonia, diffuses out through the respiratory surfaces.
The coelomic fluid contains the coelomocytes, or immune cells. There are several types of immune cells, which vary among classes and species. All classes possess a type of phagocytic amebocyte, which engulf invading particles and infected cells, aggregate or clot, and may be involved in cytotoxicity. These cells are usually large and granular, and are believed to be a main line of defence against potential pathogens. Depending on the class, echinoderms may have spherule cells, vibratile cells, and crystal cells. The coelomocytes secrete antimicrobial peptides against bacteria, and have a set of lectins and complement proteins as part of an innate immune system that is still being characterised.
Echinoderms have a simple radial nervous system that consists of a modified nerve net of interconnected neurons with no central brain, although some do possess ganglia. Nerves radiate from central rings around the mouth into each arm or along the body wall; the branches of these nerves coordinate the movements of the organism and the synchronisation of the tube feet. Starfish have sensory cells in the epithelium and have simple eyespots and touch-sensitive tentacle-like tube feet at the tips of their arms. Sea urchins have no particular sense organs but do have statocysts that assist in gravitational orientation, and they too have sensory cells in their epidermis, particularly in the tube feet, spines and pedicellariae. Brittle stars, crinoids and sea cucumbers in general do not have sensory organs, but some burrowing sea cucumbers of the order Apodida have a single statocyst adjoining each radial nerve, and some have an eyespot at the base of each tentacle.
The gonads at least periodically occupy much of the body cavities of sea urchins and sea cucumbers, while the less voluminous crinoids, brittle stars and starfish have two gonads in each arm. While the ancestors of modern echinoderms are believed to have had one genital aperture, many organisms have multiple gonopores through which eggs or sperm may be released.