Jumat, 17 Juni 2011

DEFINITION OF CORALS

A group of invertebrate marine fossils of the phylum

Cnidaria, characterized by radial symmetry and a lack of cells

organized into organs. They are related to jellyfish, hydroids,

and sea anemones, all of which possess stinging cells. Corals

are the best preserved of this phylum because they secrete a

hard calcareous skeleton. The animal is basically a simple sac

with a central mouth, surrounded by tentacles, that leads to a

closed stomach. Cnidarians are passive predators, catching

food that wanders by in their tentacles. Corals and other

cnidarians produce alternating generations of two body

forms. Medusa are forms that reproduce sexually to form

polyps that are asexual forms from which the medusa may

bud off. The corals belong to a subclass of the anthozoan

cnidarians known as the Zooantharia. The jellyfish belongs

to the Scyphozoa class, and the Hydrozoa class includes both

fresh and saltwater cnidaria dominated by the polyp stage.

Corals can live in a range of conditions from shallow

tidal pools to 19,700 feet (6,000 m) depth. They have a cylindrical

or conical skeleton secreted by the polyp stage organism,

who lives in the upper exposed part of the structure. The

skeleton is characterized by radial ridges known as septa that

join the skeleton’s outer wall (known as the theca), and may

have flat floors that were periodically secreted by the polyp.

Corals range from the Early Ordovician Tabulata forms,

joined in the Middle Ordovician by the rugose corals. They

both experienced a major extinction in the Late Devonian,

from which the rugose forms recovered stronger. Both forms

became extinct in the Early Triassic and were replaced by

modern coral forms known as Scerlactina, which apparently

arose independently from different soft-bodied organisms.

Most corals grow in colonial communities and form

reefs that provide numerous advantages including shelter for

larvae and young stages. Modern corals can survive only in

shallow waters that range in temperature from 77°F to 84°F

(25°C to 29°C), at depths of less than 300 feet (90 m). Reefs

have various forms including fringing, barrier, and atoll reefs.

See also CARBONATE; PLATFORM; REEF.

Cordillera See ROCKY MOUNTAINS.

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