Crinoidea

Crinoidea. The Crinoids or Sea Lilies are a class of Echinodermata (q.v.), which have recently been defined in the language of modern biology as caliculate, actinogonidiate, statozoic, pelmatozoic, and possibly apelmatozoic Echinoderms. All this simply means that the Crinoids are animals which belong to the group of the Echinoderms, that they generally possess a stem by which they are fixed at least at one stage of their existence, that their reproductive organs are arranged radially, and that the body is lodged in a cup formed typically of ten plates arranged in two circles. The only living Crinoid that is met with on the British coast is the "Rosy Feather Star," or Antedon. This is not, however, a good representative of the class, as it is fixed only in its larval stages while the adult is free-swimming. A typical Crinoid, such as Pentacrinus, consists of three main parts - a crown, a stem, and a root. The crown is the most important part, and contains the viscera; it consists of the calyx and the arms; the part of the calyx below the bases of the arms is known as the dorsal cup, and is typically composed of ten plates placed in two circles of five each; these are the basals and radials. In many Crinoids there is also a third ring of plates below the basals, and therefore known as underbasals; in the Eugeniacrinidiae and some others the basals are rudimentary, while in others, such as Platycrinus, the basals are large but only three in number. Above this radial ring of plates there is often a series of additional plates, of which some are placed above the radials while others occur above the angles between them; these are known as interradials. The plates above the radials are continued up to form the arms, which may be simple and five in number, or they may be any multiple of five formed by the branching of the radial series. The arms are composed of numerous small ossicles, which are grooved on the upper side; along these grooves the food is carried by currents of water to the mouth; the arms bear smaller branches known as pinnules, and in these the generative products are formed. The mouth is situated at the centre of the upper surface, and to it the five food grooves converge; the mouth opens to a short-coiled oesophagus or "gut," and the anus is also on the upper or oral surface. The mass of the soft tissues of the body enclosing the digestive system, etc., is situated within the calyx, and is known as the disc. Below it, just above the stem, is the "chambered organ," from which five cords pass up into the arms; the function of this structure was long uncertain, but it is now known to be the nerve centre, and the cords proceeding from it are the nerves. The stem is composed of a number of separate joints or ossicles united end to end; in section they are round, oval, pentagonal, or stellate; most of them are so loosely connected together as to allow the stem to be flexible; most stems bear side branches or cirri. The stem may be of great length, and in some Jurassic crinoids was as much as sixty feet high. In other cases it is short, or it may be suppressed as in Holopus or thrown off during development as in Antedon; in some it was extremely slender and wound round corals or the stems of other Crinoids. The root may be either encrusting as in the Oolitic Apiocrinus or spindle-shaped or branched, or the crinoid may anchor itself by the lowest whorl of cirri. The Crinoids are a very ancient class, as they commenced in the Cambrian system. Their stems are very commonly found as fossils, and whole masses of limestone are sometimes composed almost entirely of them; such is the Grey Carboniferous Limestone of Derbyshire. They are gregarious, and are now mainly restricted to the deep seas; and consequently were once very seldom met with. It was, therefore, generally thought that the class was becoming extinct, but later explorations have shown that they still occur in vast numbers forming large forests in the deeper seas. The Crinoids are all marine.