Crinoid
Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are members of the largest crinoid order, Comatulida. Crinoids are echinoderms in the phylum Echinodermata, which also includes the starfish, brittle stars, sea urchins, and sea cucumbers. They live in both shallow water and at depths over.
Adult crinoids are characterised by having their mouths located on their upper surface. This is surrounded by feeding arms and is linked to a U-shaped gut, with the anus being located on the oral disc near the mouth. Although the basic echinoderm pattern of five-fold symmetry can be recognised in most crinoids, the five arms are subdivided into 10 or more. These have feathery pinnules and are spread wide to gather planktonic particles from the water. At some stage in their lives, most crinoids have a short stem used to attach themselves to the substrate, but many live attached only as juveniles and become free-swimming as adults.
Only about 700 living species of crinoids are known, but the class was much more abundant and diverse in the past. Some thick limestone beds dating to the mid-Paleozoic era to Jurassic period are almost entirely made up of disarticulated crinoid fragments.
Etymology
The name "Crinoidea" comes from the Ancient Greek word κρίνον, "a lily", with the suffix –oid meaning "like".Morphology
The basic body form of a crinoid is a stem and a crown consisting of a cup-like central body known as the theca, and a set of five rays or arms, usually branched and feathery. The mouth and anus are both located on the upper side of the theca, making the dorsal surface the oral surface, unlike in the other echinoderm groups such as the sea urchins, starfish, and brittle stars, where the mouth is on the underside. Numerous calcareous plates make up the bulk of the crinoid, with only a small percentage of soft tissue. These ossicles fossilize well and some beds of limestone date from the Lower Carboniferous around Clitheroe, England, formed almost exclusively from a diverse fauna of crinoid fossils.The stem of sea lilies is composed of a column of highly porous ossicles, which are connected by ligamentary tissue. It attaches to the substrate with a flattened holdfast or with whorls of jointed, root-like structures known as cirri. Further cirri may occur higher up the stem. In crinoids that attach to hard surfaces, the cirri may be robust and curved, resembling birds' feet, but when crinoids live on soft sediment, the cirri may be slender and rod-like. Juvenile feather stars have a stem, but this is later lost, with many species retaining a few cirri at the base of the crown. Most living crinoids are free-swimming and have only a vestigial stalk. In those deep-sea species that still retain a stalk, it may reach up to in length, and fossil species are known with stems.
The theca is pentamerous and is homologous with the body or disc of other echinoderms. The base of the theca is formed from a cup-shaped set of ossicles, the calyx, while the upper surface is formed by the weakly calcified tegmen, a membranous disc. The tegmen is divided into five "ambulacral areas", including a deep groove from which the tube feet project, and five "interambulacral areas" between them. The mouth is near the centre or on the margin of the tegmen, and ambulacral grooves lead from the base of the arms to the mouth. The anus is also located on the tegmen, often on a small, elevated cone, in an interambulacral area. The theca is relatively small and contains the crinoid's digestive organs.
The arms are supported by a series of articulating ossicles similar to those in the stalk. Primitively, crinoids had only five arms, but in most modern forms, these are divided into two at ossicle II, giving 10 arms in total. In most living species, especially the free-swimming feather stars, the arms branch several more times, producing up to 200 branches in total. Being jointed, the arms can curl up. They are lined, on either side alternately, by smaller jointed appendages known as "pinnules", which give them their feather-like appearance. Both arms and pinnules have tube feet along the margins of the ambulacral grooves. The tube feet occur in groups of three of different sizes; they have no suction pads and are used to hold and manipulate food particles. The grooves are equipped with cilia that facilitate feeding by moving the organic particles along the arm and into the mouth.
Feeding
Crinoids are passive suspension feeders, filtering plankton and small particles of detritus from the sea water flowing past them with their feather-like arms. The arms are raised to form a fan shape, which is held perpendicular to the current. Mobile crinoids move to perch on rocks, coral heads, or other eminences to maximise their feeding opportunities. The food particles are caught by the primary tube feet, which are fully extended and held erect from the pinnules, forming a food-trapping mesh, while the secondary and tertiary tube feet are involved in manipulating any encountered food.The tube feet are covered with sticky mucus that traps any particles that come in contact. Once they have caught a particle of food, the tube feet flick it into the ambulacral groove, where the cilia propel the mucus and food particles towards the mouth. Lappets at the side of the groove help keep the mucus stream in place. The total length of the food-trapping surface may be very large; the 56 arms of a Japanese sea lily, with arms, have a total length of including the pinnules. Generally speaking, crinoids living in environments with relatively little plankton have longer and more highly branched arms than those living in food-rich environments.
The mouth descends into a short oesophagus. With no true stomach, the oesophagus connects directly to the intestine, which runs in a single loop right around the inside of the calyx. The intestine often includes numerous diverticulae, some of which may be long or branched. The end of the intestine opens into a short, muscular rectum. This ascends towards the anus, which projects from a small, conical protuberance at the edge of the tegmen. Faecal matter is formed into large, mucous-cemented pellets, which fall onto the tegmen and thence the substrate.
Predation
Specimens of the sea urchin Calocidaris micans found in the vicinity of the crinoid Endoxocrinus parrae have been shown to contain large quantities of stem portions in their guts. These consist of articulated ossicles with soft tissue, whereas the local sediment contained only disarticulated ossicles without soft tissue. These sea urchins very likely are predators of the crinoids, and the crinoids flee, offering part of their stem in the process.Various crinoid fossils hint at possible prehistoric predators. Coprolites of both fish and cephalopods have been found containing ossicles of various crinoids, such as the pelagic crinoid Saccocoma, from the Jurassic lagerstatten Solnhofen, while damaged crinoid stems with bite marks matching the toothplates of coccosteid placoderms have been found in Late Devonian Poland. The calyxes of several Devonian to Carboniferous-aged crinoids have the shells of a snail, Platyceras, intimately associated with them. Some have the snail situated over the anus, suggesting that Platyceras was a coprophagous commensal, while others have the animal directly situated over a borehole, suggesting a more pernicious relationship.
Water vascular system
Like other echinoderms, crinoids possess a water vascular system that maintains hydraulic pressure in the tube feet. This is not connected to external sea water via a madreporite, as in other echinoderms, but only connected through a large number of pores to the coelom. The main fluid reservoir is the muscular-walled ring canal that is connected to the coelom by stone canals lined with calcareous material. The coelom is divided into a number of interconnecting spaces by mesenteries. It surrounds the viscera in the disc and has branches within the stalk and arms, with smaller branches extending into the pinnules. It is the contraction of the ring canal that extends the tube feet. Three narrow branches of the coelom enter each arm, two on the oral side and one aborally, and pinnules. The action of cilia cause a slow flow of fluid in these canals, outward in the oral branches and inward in the aboral ones, and this is the main means of transport of nutrients and waste products. They have no heart and separate circulatory system, but a large blood vessel known as the axial organ at the base of the disc contains some slender, blind-ended tubes of unknown function, which extend into the stalk.These various fluid-filled spaces, in addition to transporting nutrients around the body, also function as both a respiratory and an excretory system. Oxygen is absorbed primarily through the tube feet, which are the most thin-walled parts of the body, with further gas exchange taking place over the large surface area of the arms. No specialised organs exist for excretion, while waste is collected by phagocytic coelomocytes.