PHYLUM COELENTERATA
(Gr. Koilos- hollow; enteron- intestine)
General Characteristics: 1.
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All are aquatic, some fresh water, mostly marine, solitary or colonial, sedentary or free- swimming.
Symmetry radial along a longitudinal oral-aboral axis.
Coelenterates are acoelomate (no body cavity)\
A low grade of tissue formation. Head and segmentation is absent.
Skeleton is of common occurrence, may be external or internal, chitinous (perisrac) or calcareous
(corals).
6. Body wall diploblastic with two cellular layers- outer ectoderm and inner endoderm with a gelatinous
acellular mesogloea in between. In advanced types mesogloea is fibrous and has cells.
7. Two types of individuals occur, attached polyps and free-swimming medusae. Some species are
notable for polymorphism, or variety of forms.
8. Mouth of polyps and bell margin of medusae often encircled by short and slender tentacles. They are
used for food-capture, intake and defense.
9. One or both body layers contain peculiar cells called nematocysts which serve for defense and food
capture.
10. The body encloses a single internal cavity, the coeleteron or the gastrovascular cavity. It serves both
for digestion and distribution of food. This cavity has a single opening, the mouth. Digestion is both
intracellular and extracellular.
11. Respiratory, circulatory and excretory systems are absent.
12. Nervous system is primitive, consisting of a diffused network of nerve cells in the body wall.
13. Sense organs include eye spots or statocysts.
14. Reproduction is both, asexual by budding or sexual by ova and sperm formation. Gonads simple
without gonoducts. Cleavage is holoblastic, the larva is a ciliated planula.
15. The life history illustrates alternation of generation or metagenesis in which sexual, free swimming,
medusoid generation alternates with an asexual polypoid generation.
Example: - Hydra, Obelia, Aurelia, Fungia, etc.
CLASSIFICATION: Coelenterates are grouped into THREE main classes.
Class I : Hydrozoa: 1. Many exhibit alternation of generations with two distinct phases in life history; the dominant polypoid
stage and the medusoid stage.
2. Body wall diploblastic made of ectoderm and endoderm, separated by a non-cellular, gelatinous
mesogloea.
3. A horny layer called perisarc is secreted by ectoderm providing rigidity and support.
4. Medusa small and with a true muscular and non-vascular velum.
5. The gastro-vascular cavity is simple and not divided by vertical partitions and mesenteries.
6. Special marginal sense organs or Rhophalia are absent.
7. Mostly colonial and marine, a few are solitary and fresh water.
Example: - Hydra, Obelia, Physalia, Halistemma.
Class II: Scyphozoa
1. Exclusively marine forms, commonly known as jelly fish.
2. Polyp phase absent or reduced, represented by a small scyphistoma producing medusa.
3. Medusa phase is dominant and medusae are large, bell or umbrella shaped and free-swimming.
4. Mesogloea extensive, gelatinous, with fibers and cells.
5. True velum is rudimentary or absent.
6. Bell margin is lobed and bears sense organs called Rhophalia.
7. Gastro-vascular system without stomodeum.
Example: - Aurelia, Rhizostoma, Cassiopeia etc.
Class III: Anthozoa
1. Polyp form alone exists, the medusa being absent.
2. Solitary or colonial, exclusively marine and attached to substratum.
3. Mesogloea contains fibers and cells.
4. Ectoderm may secrete an exoskeleton of calcium carbonate or a horny substance, often forming
massive corals. Endoskeleton may also occur.
5. Mouth enters the enteron through a tube, lined by ectoderm called stomodaeum.
6. Gastro-vascular cavity is divided by vertical partitions or septa called mesenteries, which projects
radially inwards from the body wall.
7. Individuals are mostly unisexual. Gonads are gastrodermal developing in mesenteries. Ripe ova and
sperm are discharged into the coelenteron.
Example: - Metridium (Sea- Anemone), Fungia (Mushroom coral), Favia, Acropora, Meandrina.
POLYMORPHISM IN HYDROZOA
Occurrence in the same species of more than one type of individuals, which differ in form and
function, is known as polymorphism. (Gr: polys,many + morphic, form). This ensures an efficient division
of labour between these several individuals. In coelenterates, the different individuals or zooids often get
united in the form of a colony. Thus, polymorphism is an important feature of hydrozoan colonies which
provide some of the best examples.
Two Basic forms: In Hydrozoa, there occur two main types of individuals or zooids  Polyps and Medusa.
1. Polyps: - a polyp has a tubular body with a mouth surrounded by tentacles at one end. Other end is blind
(no opening) and usually attached.
2. Medusa: - a medusa has an umbrella shaped body with marginal tentacles and a mouth, centrally located
on a position (Manubrium) of the lower concave surface.
POLYP
1) GASTROZOIDS
2) DACTYLOZOOIDS
3) GONOZOOIDS
MEDUSA
1) PNEUMATOPHORE
2) NECTOPHORE
3) BRACT
4) GONOPHORE
Polyps are concerned with feeding, protection and asexual reproduction, while medusae are
concerned with sexual reproduction. Extreme examples of polymorphism are seen in the marine, pelagic,
floating and swimming colonies of the order Siphonophora in them both polypoid and medusa forms occur
in the same colony which are specialized for feeding, reproduction, swimming or buoyancy etc.
1) In Halistemma: The colony appears as a single individual with a float at the top and a trailing stem, the stolon
bearing various kinds of zooids. The float or pneumatophore is quite small without any opening. The
epithelial float secretes gas and serves to keep the colony floating. The stem is long and differentiated into
an upper part, the nectosome bearing the float and several nectophores arranged in the base and lower part
called the siphonosome bearing the other zooids. The nectophores possess velum, musculature and
gastrovascular system, but lack the manubrium. By rhythmical contractions, they propel the colony through
the water. In the Siphonosome, the members are grouped together in assemblages termed Cormidia, which
are repeated in a linear succession.
Each Cormidium includes: 1. A leaf like hydrophyllium which partially covers the cormidium.
2. A gastrozooid with a mouth and a long tentacle with branches called tentilla, bearing numerous
nematocysts arranged in batteries serving to paralyze the prey.
3. Mouth less Dactylozooids, each with a unbranched tentacle and
4. Gonozooids which are unisexual and bear either male or female gonophores. Both kinds of
gonozooids occur is same colony.
DIAGRAMS REFER CLASS NOTES
CORALS AND CORAL REEFS
Meaning of Coral: Coral animals are marine, mostly colonial, polypoid, coelenterates, looking like miniature sea
anemones and living in a secreted skeleton of their own. Their calcareous and horny skeleton is commonly
known as coral. Some corals grow into massive, solid structures. Others form large, branched colonies.
Most of corals belong to the class Anthozoa and a few to the class Hydrozoa of phylum Coelenterata.
Structure of Coral polyp: A coral polyp is just like a sea anemone in structure except that it produces a skeleton of calcium
carbonate. A coral polyp does not have a pedal disc as its basal region is surrounded by the calcareous
skeleton.
Structure of Coral skeleton: Skeleton of a solitary coral is known as a corallite. It is a calcareous exoskeleton, secreted by
epidermis. In a colonial coral, corallites of individual polyps fuse together to form a skeletal mass called
corallum. Each corallite is like a stony cup with a basal part or basal plate and a cup wall or theca,
enclosing the aboral end of the polyp. Cavity of cup contains a number of vertical radiating ridges called
sclerosepta, proceeding from the theca towards the centre of the cup. Inner ends of the sclerosepta are
fused to form an irregular central skeleton mass or columella. A coral colony increases in size by budding
new polyps along the margin of the colony.
Corals have diverse shapes and sizes. Some are solitary and have large polyps, but majority are
colonial with very small polyps. Living polyps are found only on surface layers of coral masses. They feed
at night.when not feeding they withdraw into the cup like cavities of skeleton.
Types of corals in different groups: 1} Hydrozoan corals: - Order Hydrocorallina includes few genera, like Millepora, Stylester and
Distichopora, which are colonial and secrete massive branched calcareous exoskeleton.
2} Octocorallian corals: Order Alcyonacea includes marine, colonial and soft corals. A well known genus is Alcyonium,
popularly known as ‘dead man’s fingers’.
Order Stolonifera includes the ‘organ pipe coral’, Tubipora widely distributed on coral reefs.
Skeleton is dull red in colour due to the presence of iron salts.
Order Gorgonacea includes plant like colonies of sea fans or horny corals. In precious red coral
corallium the axial skeleton consists of spicules embedded in CaCO3 forming precious hard red corals
which is used in jewellary.
3} Hexacorallian corals: a) Solitary corals - Fungia, Flabellum etc. It is often without a theca.
b) Colonial corals- Favia, Acropora, Meandrina. Most of stony corals are colonial with plate- like, cuplike spherical or vase shaped skeleton (corallum). Stag-horn coral, Acropora has lateral branches on either
side. In Favia theca are so close together as to have common walls. In the brain coral Meandrina, polyps as
well as theca become confluent occupying valleys separated by ridges, on the surface of corallum.
CORAL REEFS
Coral colonies grow continuously in size by budding of polyps and often form extensive masses
known as coral reefs. A coral reef is a ridge or mound of lime stone, the upper surface of which is near the
surface of sea and which is formed chiefly of CaCO3 secreted by coral polyps. Principal builders of coral
reefs are stony corals (Madreporaria), but other important contributors are the Hydrocoralline and
Alcyonarians. Coralline algae and Foraminiferan, protozoa also take part in the formation of coral reefs.
Reef building corals require warm shallow waters (normally above 20ºC). They are therefore
limited to the Indo-Pacific, The central-western Pacific and Caribbean regions north of Bermuda.
A tropical coral reef is a shelter or substratum for several thousand different kinds of animalsfishes, crabs, shrimps, barnacles, worms, star-fish, brittle stars, sea cucumber, sea urchins, octopus, snails,
clams, sponges, hydroids, sea anemones and coral polyps. Perhaps no place on earth is so teaming with
such an almost incredible variety of life as a coral reef. The coral bed with its multitude organisms of
varied shapes and colours viewed through the deep-blue water, presents one of the most beautiful sights in
the world, revealing the most gorgeous flower garden.
Kinds of coral reefs
The coral reefs are of three kinds: 1} FRINGING REEFS: - The coral reefs growing in the shallow waters, closely bordering the shores of
some volcanic island or part of some continent, are termed as the fringing reef or shore reefs. They
extend from the coast a few meters to 1/3 kilometer out as a bench or platform with its surface more or less
level. The most active coral growth occurs at the outer or seaward edge of the reef platform called the reefedge or reef –front. Reef edge is broken here and there by channels through which the water falling tide
rushes off. The narrow water channel with a depth of 40-60meters, called the lagoon lies in between the
fringing reef and the shore composed of corals, sand, mud.
2} BARRIER REEF: - or encircling reefs are located some distance form the shore. The stretch of water
or lagoon separating the barrier reef from sand may be 1-7km wide. It is 20-100meters deep and suitable
for navigation of the largest ships. A barrier reef thus often encircles an island situated in the center of the
lagoon.
The world's largest notable example of a Barrier reef is The Great Barrier Reef of Australia. It is
an enormous coral structure extending along the north eastern coast of Australia for over 2,000kms of
distance from the mainland varies from 15-250kms.
3} ATOLLS or a coral island or lagoon is a ring-like circular or horse-shoe shaped reef that encloses a
central lagoon, instead of an island. The lagoon varies from few 100meters to 70-90kms in diameter and
20-90meters in depth. The rim of the ring is quite narrow, but a few hundred meters wide. It may be
complete or broken by a number of gaps or channels of which only a few are navigable. The outer side of
the atoll reef slopes off rather steeply into the depths of the ocean.
The atolls are hundreds or thousands of kilometers from the nearest land. The largest atoll of the
world is the Suvadiva of the Maldives covering of an area of 68 by 52kms, with circumference of about
195kms and not less than 102 separate little islets on its rim.
DIAGRAMS REFER CLASS NOTES