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Crinoids
Fossil range: Ordovician - Recent
Scientific classification
Kingdom: Animalia
Phylum: Echinodermata
Subphylum: Crinozoa
Class: Crinoidea
Miller, 1821
Subclasses

Articulata (540 species)
Cladida (extinct)
Flexibilia (extinct)
Camerata (extinct)
Disparida (extinct)

Crinoids, also known as sea lilies or feather-stars, are marine animals that make up the class Crinoidea of the echinoderms (phylum Echinodermata). Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". [1] They live both in shallow water and in depths as great as 6,000 meters.[citation needed]

Crinoids are characterized by a mouth on the top surface that is surrounded by feeding arms. They have a U-shaped gut, and their anus is located next to the mouth. Although the basic echinoderm pattern of fivefold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults.

There are only a few hundred known modern forms, but crinoids were much more numerous both in species and numbers in the past. Some thick limestone beds dating to the mid- to late-Paleozoic are almost entirely made up of disarticulated crinoid fragments.

Contents

Morphology

A typical crinoid fossil, showing (from bottom to top) the stem, calyx, and arms with cirri

Crinoids comprise three basic sections; the stem, the calyx, and the arms. The stem is composed of highly porous ossicles which are filled with muscular tissue. The calyx contains the crinoid's digestive and reproductive organs, and the mouth is located at the top of the dorsal cup, while the anus is located peripheral to it. The arms display pentamerism or pentaradial symmetry and comprise smaller ossicles than the stem and are equipped with cirri which facilitate feeding by moving the organic media down the arm and into the mouth.

The majority of 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 1 metre (3.3 ft) in length, although it is usually much smaller. The stalk grows out of the aboral surface, which forms the upper side of the animal in starfish and sea urchins, so that crinoids are effectively upside-down by comparison with most other echinoderms. The base of the stalk consists of a disc-like sucker, which, in some species, has root-like structures that further increase its grip on the underlying surface. The stalk is often lined by small cirri.[2]

Like other echinoderms, crinoids have pentaradial symmetry. The aboral surface of the body is studded with plates of calcium carbonate, forming an endoskeleton similar to that in starfish and sea urchins. These make the calyx somewhat cup-shaped, and there are few, if any, ossicles in the oral (upper) surface. The upper surface, or tegmen, is divided into five ambulacral areas, including a deep groove from which the tube feet project, and five interambulacral areas between them. The anus, unusually for echinoderms, is found on the same surface as the mouth, at the edge of the tegmen.[2]

The ambulacral grooves extend onto the arms, which thus have tube feet along their inner surfaces. Primitively, crinoids had only five arms, but in most living species these are divided into two, giving ten arms in total. In most living species, especially the free-swimming feather stars, the arms branch several times, producing anything up to two hundred branches in total. The arms are jointed, and lined by smaller feather-like appendages, or pinnules, which also include tube feet.[2]

Biology

Feeding

Crinoids feed by filtering small particles of food from the sea water with their feather like arms. The tube feet are covered with a sticky mucus that traps any food that floats past. Once they have caught a particle of food, the tube feet can flick it into the ambulacral groove, where the cilia are able to propel the stream of mucus towards the mouth. Generally speaking, crinoids living in environments with relatively little plankton have longer and more highly branched arms than those living in rich environments.[2]

The mouth descends into a short oesophagus. There is no true stomach, so 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.[2]

Circulatory systems

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, as in other echinoderms, but only to the body cavity. The body cavity is itself somewhat restricted, being largely replaced by connective tissue, although it is present as narrow canals within the arms and stalk.[2]

Crinoids also possess a separate haemal system, consisting of fluid-filled sinuses within the connective tissue. There is a large plexus of sinuses around the oesophagus, with branches extending down to a mass of glandular tissue at the base of the calyx.[2]

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, while waste is collected by phagocytic coelomocytes.[2]

Nervous system

The crinoid nervous system is divided into three parts, with numerous connections between them. The uppermost portion is the only one homologous with the nervous systems of other echinoderms. It consists of a central nerve ring surrounding the mouth, and radial nerves branching into the arms. Below this lies a second nerve ring, giving off two brachial nerves into each arm. Both of these sets of nerves are sensory in nature, with the lower set supplying the pinnules and tube feet.[2]

The third portion of the nervous system lies below the other two, and is responsible for motor action. This is centred on a mass of neural tissue near the base of the calyx, and provides a single nerve to each arm and a number of nerves to the stalk.[2]

Reproduction and life cycle

Crinoids are dioecious, with separate male and female individuals. They have no true gonads, producing their gametes from genital canals found inside some of the pinnules. The pinnules eventually rupture to release the sperm and eggs into the surrounding sea water.[2]

The fertilised eggs hatch to release a free-swimming vitellaria larva. The larva is barrel-shaped with rings of cilia running round the body, and a tuft of sensory hairs at the upper pole. In some cases females have been known to temporarily brood the larvae using chambers within the arms. The larva does not feed, and lasts only for a few days before settling to the bottom and attaching itself to the underlying surface using an adhesive gland on its ventral surface. The larva then metamorphoses into a stalked adult. Even the free-swimming feather stars sometimes go through this stage, with the adult eventually breaking away from the stalk.[2]

Within 10 to 16 months the crinoid will be able to reproduce.[citation needed]

Mobility

Most modern crinoids are free-swimming and lack a stem. Examples of fossil crinoids that have been interpreted as free-swimming include Marsupitsa, Saccocoma and Uintacrinus.[citation needed]

In 2005, a stalked crinoid was recorded pulling itself along the sea floor off the Grand Bahama Island. While it has been known that stalked crinoids move, prior to this recording the fastest motion of a crinoid was 0.6 meters/hour (2 ft/h). The 2005 recording showed a crinoid moving at much faster speeds.[3]

Evolution

Crinoid columnals (Isocrinus nicoleti) from the Middle Jurassic Carmel Formation at Mount Carmel Junction, Utah. Scale in mm.

The earliest known crinoids come from the Ordovician. They are thought to have evolved from primitive echinoderms known as Eocystoids. Confusingly, another early group of echinoderms were also the Eocrinoids, but that group is currently thought to be an ancestor of blastoids rather than of crinoids.

The crinoids underwent two periods of abrupt adaptive radiation; the first during the Ordovician, the other after they underwent a selective mass extinction at the end of the Permian period.[4] This Triassic radiation resulted in forms possessing flexible arms becoming widespread; motility, predominantly a response to predation pressure, also became far more prevalent.[5] After the end-Permian extinction, crinoids never regained the morphological disparity they enjoyed in the Paleozoic; they occupied a different region of morphospace, employing different ecological strategies from those that had proven so successful in the Paleozoic.[4]

The long and varied geological history of the crinoids demonstrates how well the echinoderms have adapted to filter-feeding. The fossils of other stalked filter-feeding echinoderms, such as blastoids, are also found in the rocks of the Palaeozoic era. These extinct groups can exceed the crinoids in both numbers and variety in certain horizons. However, none of these others survived the crisis at the end of the Permian period.

Fossils of passing interest

The Carboniferous crinoid, Agarocrinus americanus
Crinoid holdfasts and bryozoans on an Upper Ordovician cobble from northern Kentucky.
Root-like crinoid holdfast (Upper Ordovician, southern Ohio).

Some fossil crinoids, such as Pentacrinites, seem to have lived attached to floating driftwood and complete colonies are often found. Sometimes this driftwood would become waterlogged and sink to the bottom, taking the attached crinoids with it. The stem of Pentacrinites can be several metres long. Modern relatives of Pentacrinites live in gentle currents attached to rocks by the end of their stem, which is fairly short. The largest fossil crinoid on record had a stem 40 m (130 ft) in length.[6]

In 2006, geologists isolated complex organic molecules from 350-million-year-old fossils of crinoids—the oldest such molecules yet found. Christina O'Malley, a doctoral student in earth sciences at The Ohio State University, found orange and yellow organic molecules inside the fossilized remains of several species of crinoids dating back to the Mississippian period.[7]

Crinoid uses in culture

References

  1. ^ Webster's New Universal Unabridged Dictionary. 2nd ed. 1979.
  2. ^ a b c d e f g h i j k l Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 997-1007. ISBN 0-03-056747-5. 
  3. ^ Baumiller, Tomasz K. and Messing, Charles G. (6 October 2005). "Crawling In Stalked Crinoids: In Situ Observations, Functional Morphology, and Implications for Paleozoic Taxa". Geological Society of America Abstracts with Programs, Vol. 37, No. 7. pp. 62. 
  4. ^ a b Foote, M. (1999). "Morphological diversity in the evolutionary radiation of Paleozoic and post-Paleozoic crinoids" (PDF). Paleobiology 25 (sp1): 1–116. doi:10.1666/0094-8373(1999)25[1:MDITER2.0.CO;2]. http://www.jstor.org/stable/pdfplus/2666042.pdf. Retrieved 2008-05-12. 
  5. ^ Baumiller, T. K. (2008). "Crinoid Ecological Morphology". Annual Review of Earth and Planetary Sciences 36: 221–249. doi:10.1146/annurev.earth.36.031207.124116.  edit
  6. ^ Ponsonby, Dr. David; Prof. George Dussart (2005). The Anatomy of the Sea. Vancouver: Raincoast Books. pp. 129. ISBN 0-8118-4633-4. 
  7. ^ Oldest Complex Organic Molecules Found in Ancient Fossils Newswise, Retrieved on August 4, 2008.
  8. ^ "Identifying Unknown Fossils (by their shape)". Kentucky Geological Survey / University of Kentucky. http://www.uky.edu/KGS/fossils/fossilid.htm. Retrieved 2009-06-21. 
  9. ^ "Office of the Secretary of State, Missouri". http://www.sos.mo.gov/symbols/symbols.asp?symbol=fossil. 

See also

External links


Simple English

Redirecting to Crinoidea








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