Brittle Stars - Ophiuroidea - The Animal Encyclopedia

Brittle Stars - Ophiuroidea - The Animal Encyclopedia Brittle stars (Ophiuroidea) are a group of echinoderms that resemble starfish. There are about 1500 species of brittle stars alive today and most species inhabit marine habitats with depths greater than 1500 feet. There are a few species of shallow water brittle stars. These species live in the sand or mud just below the low tide mark. They often live amongst coral and sponges as well. Brittle stars inhabit all the worlds oceans and live in a variety of climate regions including tropical, temperate and polar waters. Brittle stars are subdivided into two basic groups, the brittle stars (Ophiurida) and the basket stars (Euryalida). Brittle stars have a star shaped body. Like many echinoderms, they exhibit pentaradial symmetry, a 5-sided radial symetry. Brittle stars have five arms that join together at a central body disk. The arms are clearly delineated from the central body disk, and in this way brittle stars can be distinguished from starfish (starfish arms blend with the central body disk such that it is not easy to delineate where the arm ends and the central body disk begins). Brittle stars move using a water vascular system and tube feet. Their arms can move side to side but not up and down (if they are bent up or down they break, hence the name brittle star). Their arms are extremely flexible from side to side and enable them to move through the water and along substrate surfaces. When they move, they do so in a straight line, with one arm serving as the forward directing point and other arms pushing the body along that path. Brittle stars and basket stars both have long flexible arms. These arms are supported by calcium carbonate plates (also known as vertebral ossicles). The ossicles are encased in soft tissue and jointed plates that run the length of the arm. Brittle stars have a nervous system that consists of a nerve ring and that encircles their central body disk. Nerves run down each arm. Brittle stars, like all echinoderms, lack a brain. The have no eyes and their only developed senses are chemosensory (they can detect chemicals in the water) and touch. Brittle stars undergo respiration using bursae, sacks that enable gas exchange as well as excretion. These sacs are located on the bottom of the central body disk. Cilia within the sacs direct water flow so that oxygen can be absorbed from the water and waste flushed from the body. Brittle stars have a mouth that has five jaw-like structures around it. The mouth opening is also used to expel waste. An esophagus and stomach connect to the mouth opening. Brittle stars feed on organic material on the sea floor (they are primarily detritivores or scavengers although some species occasionally feed on small invertebrate prey). Basket stars feed on plankton and bacteria they catch by suspension feeding. Most species of brittle stars have separate sexes. A few species are either hermaphroditic or protandric. In many species, larvae develop inside the body of the parent. When an arm is lost, brittle stars often regenerate the lost limb. If a predator catches a brittle star by its arm, it loses the arm as a means of escape. Brittle stars diverged from other echinoderms about 500 million years ago, during the Early Ordovician. Brittle stars are most closely related to sea urchins and sea cucumbers. Details about the evolutionary relationship of brittle star to other echinoderms are not clear. Brittle stars reach sexual maturity at about 2 years of age and become full grown by 3 or 4 years of age. Their life span is generally about 5 years. Classification: Animals Invertebrates Echinoderms Brittle Stars

Twelver Shiites, or Ithna Ashariyah

Twelver Shiites, or Ithna Ashariyah Twelver Shiites, known in Arabic as IthnÄ  ‘AsharÄ «yah, or ImÄ miyÄ h (from Imam), form the principal branch of Shiite Islam and are sometimes synonymous with Shiitism, even though factions such as the IsmÄ Ã„ «lÄ «yah and ZaydÄ «yah Shiites do not subscribe to Twelver doctrine.   Alternate spellings include  IthnÄ  ‘AsharÄ «yah,  ImÄ miyÄ h,  and ImamiyÄ . Twelvers are the followers of the 12 imams they consider to be the only rightful successors of the Prophet Muhammad, beginning with Ali ibn Abu Talib (600-661 CE), Muhammads cousin and son-in-law, and ending with Muhammad ibn al-Hasan (born 869 CE), the 12th imam whoaccording to Twelver beliefwill emerge and bring peace and justice to the world, becoming the ultimate savior of humankind (Muhammad never appeared publicly and is currently considered in major occultation as the Mahdi). Sunnis recognize Ali as the fourth caliph, but founding commonalities between Sunnis and Shiites end with him. Some Muslims have never recognized the first three as legitimate caliphs, thus forming the nucleus of Islams protesting Shiites. The seeming subversion never sat well with Sunnis, whose habit it became to mercilessly and brutally persecute Alis followers and assassinate subsequent imams, most spectacularly among those the killing in the battle of Hussayn  (or Hussein) Ibn Ali, the third Imam (626-680 CE), on the plains of Karbala. The killing is most famously commemorated in the annual rituals of Ashura. The copious bloodletting gave Twelvers their two most prominent characteristics, like birthmarks on their creed: a cult of victimology, and a cult of martyrdom. The Safavid Dynasty Twelvers never had an empire of their own until the Safavid dynastyone of the most remarkable dynasties ever to have ruled Iranwas established in Iran in the 16th century and the Qajar dynasty in the late 18th century when Twelvers reconciled the divine and the temporal in the leadership of the reigning imam. Ayatollah Ruhollah Khomeini, through his 1979 Islamic Revolution in Iran, pushed the fusion of the temporal and the divine furthest, adding a layer of ideological expediency under the banner of Supreme Leader. A strategic revolutionary, in the words of writer Colin Thubron, Khomeini created his own Islamic state above Islamic law. Twelvers Today The majority of Twelverssome 89%reside in Iran today, with other large populations existing but being mightily oppressed in Azerbaijan (60%), Bahrain (70%), and Iraq (62%). Twelvers make up some of the most destitute populations in countries such as Lebanon, Afghanistan, and Pakistan as well. The three major legal schools of Twelver Shia Islam today include the Usuli (the most liberal of the three), the Akhbari (who rely on traditional religious knowledge), and the Shayki  (at one time absolutely apolitical, the Shaykis have since become active in the Basra, Iraq, government as its own political party).