- Corals are developed by the coelenterate organisms.
- Corals are a deposit of calcium carbonate.
- The coelenterate animal produces calcareous skeleton.
- But most coral producing polyps belong to the class Anthozoa.
- The true stony corals belong to the order Madreporaria of class Anthozoa. Many Solitary and colonial anthozoans produce corals which are sometimes brilliantly colored.
- The corals develop into coral rocks and
Coral producing Polyp Structure:
- A coral-polyp resembles sea-anemone in its shape.
- The coral is its external shell.
- It is a product of the ectodermal cells, called Calicoblasts.
- The formation of the coral is not clearly followed, so far Calcareous crystals may be precipitated in the matrix and secreted outside the epidermis.
- Thus outer protective shell may be formed.
- A coral polyp will not show a pedal disc as its basal part is surrounded by the calcareous skeleton.
- The oral disc bears tentacles in cycles of 6.
- The circular mouth leads into a short stomodeaum which will not show siphonoglyphs.
- The mesenteries are restricted to the upper part of the polyp.
- The muscles are poorly developed.
FORMATION OF CORALS
- The basal region of the coral polyp is fixed in a cup-shaped calcareous exoskeleton secreted by the epidermis of the base.
- On this basal plate a large number of radially arranged vertical septa are formed.
- They grow in height and they push the ectoderm up.
- An external wall is formed because of the circular up growth of the plate.
- The single cup like exoskeleton formed by an individual polyp is called a coralitte.
- It has the shape of the polyp.
- The majority of the corals are colonial, and the skeleton of an entire colony is termed coralium it may contain thousands of corallites.
Example for Coral formation
- In "Flabellum" the coral formation is based on these lines.
- The corallite is disc like. It is 5 mm to 25 cm. in length.
- The outer wall of the cup is made by stony calcium carbonate.
- It is called theca.
- The flattened bottom of the cup beneath the polyp is called the basal plate. The cavity of the cup develops a number of vertical septa or sclero-septa, proceeding form theca towards the center of the cup.
- Like mesenteries, they are typically arranged in cycles of six (6 primaries extending towards the center, 6 secondary’s, 12 tertiaries, 24 quartemaries etc.)
- The sclerosepta alternates with mesenteries Corallite lies entirely outside the polyp body.
- The polyp body is pushed up into ridges over the sclerosepta.
- Each ridge is covered by an internal portion of the body-wall Thus, the sclerosepta are external and lie outside the enteric cavity.
- Between the sclerosepta and the mesenteries, the polyp base is depressed into pockets called loculi.
In many corals, the theca is covered by a second calcareous wall called epitheca. The space separating the epitheca and theca, will show projections called costae.
Types of Corals and Examples: The true corals belong to Anthozoa but in class hydrozoa also some corals are seen.
I. Hydrozoan Corals :
- It is a massive, calcareous skeleton with two types opening
- a) Large gastropores, b) Smaller dactylopores.
Millipora is called stinging coral because it is the only coral with nematocysts, which causes pain to man.
2. Stylasterina :
- It is found in warm tropical water.
- The colony is tree like. It is also similar to millepora but has cup like gastropores. There is a pointed style in the centre of each cup.
II.Authozoan Corals :
1.Alcyonium (dead man's fingers) :
- It is a marine colonial form living attached to stones.
- It is tree like. It grows to 4 to 10 cm height.
- The simplest form of skeleton is seen in this animal, which contains minute calcareous spicules.
2. Tubipora (organ-pipe coral) :
- It is a marine colony distributed in warm waters The spicules are fused to form a tube around the polyp.
- Many such tubes are united by horizontal platforms Again new polyps are formed They develop vertical tubes.
- This process is repeated. A big coral is formed.
3. Corallium (red coral):
- It is a marine sedentary form.
- It grows to 30 cm. It is highly branched.
- The polyps are white in colour. Autozooids'and siphonozooids are present.
- Herd and branched skeletal axis is formed by the union of many calcareous spicules in a matrix.
- Thus a coral is developed which is red in colour.
4. Gorgonia (Sea fan) :
- It is also called sea-whip.
- It is seen in tropical waters.
- It is tree like colony, it grows 80 cm. height.
- A central, rod like branched skeleton is formed by the ectoderm.
- Mesogloea also contains many calcareous spicules.
III.The corals belong to Madreporaria are true corals.
- The skeleton is entirely calcareous and is secreted by the ectoderm.
- The following are good examples.
1.Fungia (Mushroom coral) :
- It is marine form. The coral is discoid.
- The septa are numerous and are connected by small calcareous rods called synapticula.
- This coral is true coral.
2. Meandrina(Or Brain coral) :
- It is very big and grows up to 8 feet in diameter.
- It weighs in tons.
- The surface of the colony is marked by long curved grooves; hence it looks like human brain.
- In a living brain coral the polyps do not occupy separate cups.
- The mouths of these compound polyps will be separate.