In this article we will discuss about Coelenterata:- 1. History of Coelenterata 2. General Characters of Coelenterata 3. Classification 4. Tissues 5. Polyp and Medusa 6. Polymorphism 7. Corals.
Contents:
- History of Coelenterata
- General Characters of Coelenterata
- Classification of Coelenterata
- Tissues of Coelenterata
- Polyp and Medusa of Coelenterata
- Polymorphism of Coelenterata
- Corals of Coelenterata
Contents
1. History of Coelentrata:
Aristotle knew the stinging qualities of coelenterates and considered these organisms as intermediate between plants and animals and termed them Acalephae or Cnidae (Gr., akalephe = nettle; cnidos = thread). They were long included in the Zoophyta (Gr., zoon = animal; phyton = plant) together with various forms from sponges to ascidians.
The animal nature of coelenterates was established by Peyssonel (1723) and Trembley (1744). Linnaeus, Lamarck and Cuvier grouped the coelenterates under Radiata which included the echinoderms also because of their symmetry. Finally, Leuckart (1847) separated the coelenterates from echinoderms and created a separate phylum Coelenterata (Gr., koilos = cavity; enteron – intestine).
However, the Coelenterata of Leuckart also included the sponges and ctenophores.
Hatschek (1888) splitted Leuckart’s Coelenterata into three distinct phyla – Spongiaria (Porifera), Cnidaria (Coelenterata) and Ctenophora.
The coelenterates or cnidaria are distinguished from sponges in being “tissue animals” (Metazoa) that have distinct digestive cavity. The coelenterates differ from ctenophores in being primarily radial in symmetry, in possessing nematocysts, in having a polyp stage, and in reproducing both asexually and sexually.
Coelenterata or Cnidaria are radially symmetrical diploblastic animals with only epidermis and gastrodermis, between these two layers is a jelly-like mesogloea which is originally structure less but in higher forms, it becomes fibrous and has wandering amoebocytes.
Epidermis and gastrodermis are in two layers, each made of a variety of cells showing a division of labour, the cells form poorly organised body tissues. All functions of the body are performed by tissues and never by organs.
The radially symmetrical body has an oral-aboral axis, there is a single coelenteron or gastro vascular cavity which has only one aperture, the mouth. The mouth is used both for ingestion and egestion. There is no coelom. They bear tentacles and nematocysts. The nervous system is a primitive nerve net. They may have an exoskeleton, and in some there is an endoskeleton.
There is an oval ciliated planula larva during development. Coelenterata include some 9,000 living species and there are many known fossil forms dating back to the Ordovician period. Except for a few species of freshwater forms almost all Coelenterata are marine. They include hydras, jelly-fishes, sea anemones, and corals. The phylum is divided into three classes, namely, Hydrozoa, Scyphozoa and Anthozoa.
2. General Characters of Coelenterata:
1. Coelenterates are Metazoa or multicellular animals with tissue grade of organisation.
2. They are aquatic, mostly marine except few freshwater forms like Hydra.
3. They are sedentary or free-swimming and solitary or colonial.
4. Individuals are radially or bi-radially symmetrical with a central gastro vascular cavity communicating to the exterior by the mouth.
5. They are diploblastic animals; body wall consists of an outer layer of cells called ectoderm and inner layer of cells the endoderm cemented together by an intermediate layer of non-cellular gelatinous mesogloea.
6. Acoelomate animals because they do not possess a second body cavity, the coelom.
7. Short and slender tentacles encircle the mouth in one or more whorls.
8. The tentacles are provided with nematocysts; tentacles serve for food capture, its ingestion and for defence. These are also present on body layers, these are adhesive organs.
9. They exhibit the phenomenon of polymorphism with very few exceptions; the main types of zooids in polymorphic forms are polyps and medusa. Polyp is sessile and asexual zooid, while medusa is free-swimming and sexual zooid.
10. Skeleton, either exoskeleton or endoskeleton is of common occurrence.
11. They are usually carnivorous; digestion is extracellular as well as intracellular. Anus is not found.
12. Nervous system consists of one or more networks or nerve-cells and neurites located in the ectoderm and endoderm.
13. Respiratory, circulatory and excretory systems are wanting.
14. Reproduction is both by asexual and sexual methods.
15. Asexual reproduction occurs by budding and sexual reproduction by the formation of gametes.
16. A ciliated planula larva usually present in the life history.
17. The life history exhibits the phenomena of alternation of generations or metagenesis in which the asexual polypoid, sessile generation alternates with sexual medusoid, free-swimming generations.
3. Classification of Coelenterata:
The classification followed here is given by Hyman, L.H,. (1940). According to Hyman, Phylum Coelenterata has been divided into three classes, viz., Hydrozoa, Scyphozoa and Anthozoa.
Class I. Hydrozoa:
(Gr., hydra = water; zoios = animal):
1. Hydrozoa are solitary and freshwater or mostly colonial and marine, sessile and free- swimming forms.
2. They exhibit tetramerous or polymerous radial symmetry.
3. Body wall consists of an outer ectoderm and an inner endoderm separated by non- cellular mesogloea.
4. Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament.
5. Skeleton or horny structure is horny peri- sarc in some forms, while coenosarc secretes a skeleton of calcium carbonate forming massive stony structure or coral in other forms.
6. They exhibit polymorphism, There are two main types of zooids, the polyp and medusa.
7. Medusa is provided with true muscular velum.
8. Many Hydrozoa exhibit alternation of generations.
9. Reproductive products or sex cells are usually ectodermal in origin and discharged externally.
10. Cleavage is holoblastic, embryo ciliated planula.
Order 1. Hydroida:
1. Solitary or colonial forms.
2. Polypoid generation well developed gives rise to free or abortive medusae by budding.
3. Sense organs of medusae are ocelli and statocysts and exclusively ectodermal in origin.
Suborder 1. Anthomedusae or Athecata:
1. Solitary or colonial.
2. Polyps are not enclosed in hydrothecae and the medusae are also naked without gonothecae.
3. Medusae are tall, bell-like, bearing gonads on the manubrium having strongly arched umbrella.
4. Medusae bear eye spots or ocelli at the bases of tentacles.
5. Statocysts are absent.
Examples:
Hydra, Tubularia, Bougainvil- lea, Hydractinia, Eudendrium, Pennaria.
Suborder 2. Leptomedusae or Thecata:
1. Colonial Hydrozoa.
2. Polyps are enclosed in hydrotheca and medusae are covered with gonothecae.
3. Free medusae are flattened, bowl or saucer- shaped, bearing gonads on the radial canals.
4. Medusae usually bear statocysts.
5. Eye spots or ocelli are absent
Examples:
Obelia, Sertularia, Plumularia, Campanularia.
Order 2. Milleporina:
1. Colonial coral-like Hydrozoa without perisarc.
2. Massive calcareous skeleton is secreted by ectoderm provided with pores through which polyps protrude out.
3. Colony have two kinds of zooids, the gastrozooid and the dactylozooid.
4. Gastrozooids (nutritive zooids) are short provided with mouth and tentacles.
5. Dactylozooids are elongate, hollow, slender with tentacles but without mouth.
6. Medusae develop in small chambers, becoming free, devoid of mouth, radial canals and tentacles.
Example:
Millepora.
Order 3. Stylasterina:
1. Colonial coral-like Hydrozoa colony have two kinds of zooids, the dactylozooids and gastrozooids.
2. Dactylozooids are small, solid without tentacles.
3. Gastrozooids have a cup with pointed spine.
4. Medusae develop in special cavities reduced to sporosacs.
5. Larva is liberated as planula.
Example:
Stylaster.
Order 4. Trachylina:
1. Polypoid stage reduced or absent.
2. Medusae are large, provided with tentaculocysts, statocysts and lithocysts enclosed in the endoderm.
Suborder 1. Trachymedusae:
1. Margin of the umbrella is smooth.
2. Manubrium is long.
3. Gonads borne on the radial canals.
Example:
Geryonia.
Suborder 2. Narcomedusae:
1. Margin of the umbrella is scalloped or clefted by tentacle bases.
2. Manubrium is short.
3. Gonads borne in the floor of the stomach.
Examples:
Cunina, Solmaris.
Order 5. Siphonophora:
1. Siphonophora are polymorphic, free swimming or floating colonial Hydrozoa.
2. Colony consists of several types of polypoid and medusoid individuals attached to stem or disc.
3. Polyps without tentacles.
4. Medusae always incomplete and rarely freed.
Suborder 1. Calycophora:
1. The upper end of the colony is provided with one or more swimming bells.
2. Apical float or pneumatophore absent.
Example:
Diphyes.
Suborder 2. Physophorida:
1. Upper end of the colony bears a float or pneumatophore.
Examples:
Physalia, Velella, Porpita, Halistemma.
Class II. Scyphozoa (Gr., skyphos = cup; zoios = animal):
1. Scyphozoa include large jelly-fishes or true medusae are exclusively marine.
2. Medusae are large, bell or umbrella-shaped, without true velum, free swimming or attached by an aboral stalk.
3. Marginal sense organs are tentaculocysts having endodermal statoliths.
4. Polypoid generation absent or represented by small polyp, the scyphistoma which gives rise to medusae by strobilisation or transverse fission.
5. Gastrovascular system without stomodaeum with gastric filaments and may or may not be divided into four inter-radial pockets by septa.
6. Mesogloea is usually cellular.
7. Gonads are endodermal and the sex cells are discharged.
Order 1. Stauromedusae or Lucernaridae:
1. Sessile, sedentary Scyphozoa attached by an aboral stalk.
2. Body globet or trumpet shaped.
3. Mouth cruciform (four cornered) with small oral lobes and a short quadrangular manubrium.
4. Gastrovascular system is divided into central stomach and four per-radial pouches by the four inter-radial septa.
5. Gonads are elongated band-like borne on the faces of septa.
6. Marginal sense organs absent.
7. Fertilisation is external.
8. Larva is planula without cilia.
Examples:
Lucernaria, Haliclystus.
Order 2. Cubomedusae or Carybdeida:
1. Free-swimming Scyphozoa found in warm and shallow waters of tropical and subtropical regions.
2. Body cubical with four flat sides.
3. Four hollow inter-radial tentacles borne on the margin of the sub-umbrella.
4. Four per-radial tentaculocysts or rhopalia are present.
5. Each tentaculocyst is provided with a lithocyst and one or more ocelli.
6. Mouth is cruciform and gastric pouches are present.
7. Gonads are leaf-like.
Examples:
Charybdaea, Tamoya.
Order 3. Coronate:
1. Free-swimming scyphomedusae found inhabiting the deep waters of ocean.
2. Body conical, dome-shaped or flattened, grooved.
3. The umbrella is divided by a coronal groove (horizontal furrow) into an upper cone and a lower crown.
4. The crown consists of pedal lobes, pedalia.
5. The pedalia bear solid tentacles.
6. The bell margin is scalloped into lappets alternate with pedalia.
7. Mouth is cruciform.
8. Tentaculocysts are four to sixteen.
Examples:
Pericolpa, Periphylla.
Order 4. Semaeostomeae:
1. Most common free-swimming medusae found inhabiting the coastal waters of all oceans.
2. The umbrella is flat, saucer or bowl-shaped.
3. Mouth is square.
4. The corners of the mouth produce four oral arms which are grooved with frilled edges.
5. The margin of the umbrella is fringed with hollow tentacles.
6. Eight or more tentaculocysts are present.
7. Gastric pouches and filaments are absent.
Examples:
Aurelia, Cynaea.
Order 5. Rbizostomae:
1. Free-swimming Scyphozoa found in shallow waters of tropical and subtropical oceans.
2. The umbrella is saucer or bowl-shaped or flattened or even concave on the top.
3. Mouth is surrounded by eight oral arms, bearing numerous funnel-shaped mouth on their edges.
4. Marginal tentacles are absent but 8 or more tentaculocysts are present.
5. Four sub-genital pits are generally present.
Examples:
Rhizostoma or Pilema, Cassiopeia.
Class III. Anthozoa: (Gr., anthos = flower; zoios = animal):
1. Solitary or colonial exclusively marine forms.
2. They are exclusively polypoid.
3. Medusoid stage is altogether absent.
4. Body usually cylindrical with hexamerous, octomerous or polymerous biradial or radio bilateral symmetry.
5. The oral end of the body is expanded radially into an oral disc bearing hollow tentacles surrounding the mouth in the centre.
6. The stomodaeum is present, often provided with one or more ciliated grooves the siphonoglyphs.
7. Gastro vascular cavity is divided into compartments by complete or incomplete septa or mesenteries.
8. Mesenteries bear nematocysts at their inner free edges.
9. Mesogloea contains fibrous connective tissue and amoeboid cells.
10. Skeleton either external or internal.
11. Exoskeleton is formed from calcium carbonate which often form massive corals.
12. Nervous system is in the form of typical nerve net without a concentrated central nervous system.
13. Gonads are endodermal, develop in the mesenteries.
14. The ripe sexual products are discharged into coelenteron.
15. Fertilisation is external.
16. The fertilised egg develops into a planula larva, which after a short free life settles down and develops into an adult.
Subclass 1. Alcyonaria or Octocorallia:
1. Colonial marine forms.
2. Polyps are long or short cylinder terminating orally into a flat circular oral disc having the oval or elongated mouth in the centre.
3. Polyps always bear eight pinnate, hollow tentacles.
4. Eight complete mesenteries are present.
5. Single ventral siphonoglyph is present.
6. Endoskeleton is the product of mesogleal cells comprised calcareous spicules either calcareous or horny in nature.
7. Polyps are dimorphic in some forms.
Order 1. Stolonifera:
1. The Stolonifera are inhabitants of shallow waters in the tropical and temperate regions.
2. Polyps arise singly from a creeping base and are connected by one or more solenial tubes.
3. Skeleton consists of loose spicules or of compact tubes and platforms.
Examples:
Tubipora, Clavuldria.
Order 2. Telestacea:
1. Colony consists of simple or branched stems arising from a creeping base.
2. Each stem is very elongated polyp bearing lateral polyps.
3. Skeleton consists of spicules fused by calcareous or horny secretions.
Example:
Telesto.
Order 3. Alcyonacea:
1. Colony mushroom-shaped or branched into stout blunt processes.
2. Lower part of the polyp fused into a fleshy mass with only oral ends protruding.
3. Polyps are dimorphic in some forms bearing autozooids and siphonozooids.
4. Skeleton consists of separate calcareous spicules, not axial.
Examples:
Alcyonium, Xenia.
Order 4. Coenothecalia:
1. Skeleton is massive, composed of crystalline calcareous fibres of calcium carbonate, not of fused spicules.
2. Skeleton is perforated by numerous larger and smaller erect cavities. The larger ones contain the lower ends of the polyps, while the smaller ones the erect terminal tubes.
3. Commonly known as blue corals found on the coral reefs in the Indo-Pacific.
Example:
Heliopora.
Order 5. Gorgonacea:
1. The colony is usually plant-like, consists of a main stem arising from basal plate or tuft of stolon’s and number of branches bearing polyps.
2. The axial skeleton composed of calcareous spicules or horn-like material, gorgonin is present.
3. Commonly known as sea fans, sea feathers and sea whips are found in tropical and subtropical shores.
Examples:
Gorgonia, Corallium.
Order 6. Pennatulacea:
1. The colony is elongated and divided into a proximal stalk or peduncle and a distal rachis.
2. Peduncle is embedded in the mud and sand.
3. Rachis is the axial polyp bears numerous dimorphic polyps on its lateral branches.
4. The main stem is supported by calcareous or horny skeleton.
Examples:
Pennatula, Renilla, Cavernu- laria, Pteroides.
Subclass II. Zoantharia or Hexacorallia:
1. Solitary or colonial marine forms.
2. Tentacles simple, rarely branched hollow cone-shaped, numerous arranged in the multiple of five and six but never eight.
3. Mesenteries are numerous arranged in the multiple of five or six, may be complete or incomplete.
4. Two siphonoglyphs are commonly present.
5. Endoskeleton when present is calcareous, derived from ectoderm.
6. Polyps are usually monomorphic.
Order 1. Actiniaria:
1. Solitary or colonial.
2. Body cylindrical divided into oral disc, column and base.
3. The aboral end is also provided with a pedal disc.
4. Tentacles and mesenteries are numerous and often arranged in the multiple of six.
5. Siphonoglyph is one or more.
6. Skeleton absent.
Examples:
Actinia, Metridium, Adamsia, Edwardsia.
Order 2. Madreporaria:
1. Mostly colonial rarely solitary form.
2. Exoskeleton is hard and calcareous secreted by the ectoderm.
3. Polyps are small enclosed in the cup-like cavities of the exoskeleton.
4. Siphonoglyph is usually absent.
Examples:
Astraea, Fungia, Favia, Madrepora, Meandrina.
Order 3. Zoanthidea:
1. Mostly colonial some times solitary forms.
2. Skeleton and pedal disc absent, but the body wall may contain calcareous bodies.
3. Polyps are generally small.
4. Mesenteries are paired. A pair composed of one complete and one incomplete mesentery.
5. Only one ventral siphonoglyph present.
Example:
Zoanthus.
Order 4. Antipatharia:
1. Plant-like colonial forms found in the deep waters in the oceans.
2. The lower end of the colony usually consists of a basal plate for the attachment with some objects.
3. Skeleton in the form of a branched chitinoid axis derived from the ectoderm.
4. The axial skeleton bears the polyps which are dioecious but the colony may be hermaphrodite.
5. Mesenteries and tentacles are 6-24 in number.
6. Two siphonoglyphs present.
Example:
Antipathes.
Order 5. Ceriantharia:
1. Long solitary anemone-like forms living in the vertical cylindrical cavities in the sea bottom.
2. Body smooth cylindrical and elongated with an oral disc.
3. Pedal disc and skeleton absent.
4. Tentacles are simple numerous and of two types, marginal and oral.
5. Mesenteries are numerous, single and complete.
6. Only single dorsal siphonoglyph present.
Example:
Cerianthus.
4. Tissues of Coelenterata:
The tissues of Coelenterata consist of ectoderm and endoderm, each forming a single layer of a variety of cells, and Coelenterata are regarded as animals at the tissue level of organisation. Together the cells form a thin body wall surrounding an enteron cavity. Increase in thickness and complexity of the body wall is made possible by the development of an intermediate gelatinous layer called mesogloea.
The polyps of lower Coelenterata have a very thin mesogloea, but in polyps of Anthozoa the mesogloea contains fibres and cells and it has become much thicker; in the larger medusae it is very thick and firm, it has become like an internal skeleton and it also stores food.
The chief type of cell found in lower Coelenterata is a columnar epitheliomuscular cell produced into muscle fibres which are embedded in the mesogloea. Such a cell is like a protozoan because its parts carry on different functions.
The endodermal cells can form flagella for churning up the fluid in the enteron, or the flagella can be withdrawn and pseudopodia formed by the same cells for engulfing food into food vacuoles, the same cells also form muscle processes for contraction.
Thus, all the functions of the body are carried out by tissues and never by organs though specialised structures, such as tentaculocysts, may be regarded as organs, but in general the Coelenterata have reached only a tissue-grade of organisation.
5. Polyp and Medusa of Coelenterata:
The polyp leads an easy sedentary life, it has a fixed cylindrical body with a comparatively thin body wall, the free distal end has a hypostome with an apical mouth leading into an enteron, below the hypostome is a circle of extensible tentacles directed upwards and armed with nematocysts, the tentacles may be hollow containing an extension of the enteron, the proximal end of the polyp is closed.
The medusa leads a freely drifting life which requires an elaborate structure and physiological development. Great widening of the body has occurred, especially along the oral surface. The enteron is restricted to a central gastric cavity and canals, this is due to an increase in thickness of the mesogloea on the aboral side which pushes the two endoderm layers together to form a double layered endoderm lamella.
The hypostome has become the manubrium whose rim has become wide and bent downward to form a bell, this has pushed the tentacles to the margin of the bell, tentacles hang downwards, and at times a second set of tentacles may develop on the manubrium.
The muscular system of the medusa is highly developed because the muscular portion of epitheliomuscular cells increases to form elongated striated muscle fibres and the epithelial part diminishes. The nervous system is diffuse, as in the polyp, but in some medusae it becomes concentrated to form a nerve ring and in connection with this well defined sense organs are formed.
6. Polymorphism of Coelenterata:
Polymorphism (Gr., poly = many; morphe = form) is the occurrence of several different types of individuals or zooids in a single species during its life cycle or as members of the colony, the members perform different functions so that there is a division of labour amongst the members.
Coelenterata are noted for their polymorphism, but the various types are reducible to either a polypoid or medusoid type. The polyp and medusa occur in a number of morphological variations. However, polymorphism may be defined as the representation of a single organism by more than one kind of individuals or zooids which differ in their form and function.
Patterns of Polymorphism:
Most Hydrozoa exhibit dimorphism, there is a nutritive polyp or gastrozooid which is sessile, it has a mouth, tentacles and an enteron cavity for food.
The second individual is a medusa which is jelly-like, saucer-shaped, with tentacles on the margin, it is free-swimming and bears gonads. Alcyonaria have only polyps, but they are dimorphic, a nutritive and reproductive polyp called autozooid which has tentacles, gonads and mesenteries; the other polyp is a siphonozooid with no tentacles or gonads, it only maintains a circulation of water in the canals of the colony.
Some hydrozoan colonies are trimorphic, besides the nutritive polyps and reproductive medusae, they also have modified polyps called gonozooids or blastostyles, often enclosed in a chitinous gonotheca, they have no mouth or tentacles, and they produce medusae or their morphological equivalents by budding.
Siphonophora show the highest degree of polymorphism which is not found anywhere else in the animal kingdom. They may have three kinds of polypoid zooids and four kinds of medusoid zooids.
The Polypoid Zooids are as follows:
1. Gastrozooids are tubular or saccular with a mouth, and there may be one tentacle bearing nematocysts. They are used for digestion and ingestion of liquid food. Usually a single long and contractile hollow tentacle arises from the base of gastrozooid which bears numerous fine lateral contractile branches called tentilla. The tentilla has knob or coil of nematocysts.
2. Dactylozooids are for defence and obtaining food, they are tubular with no mouth, with a long tentacle armed with batteries of nematocysts. Modified dactylozooids associated with gonophores on gonozooids are termed gonopalpons. The dactylozooids are also called palpons or feelers.
3. Gonozooids may be like gastrozooids or they may be branching blastostyles bearing clusters of degenerate male and female medusae which produce germ cells for reproduction. Mostly, the gonozooids take the form of a branched stalk called gonodendron having tufts of gonophores with gonopalpons.
The Medusoid Zooids are as follows:
1. Pneumatophore is a gas-filled vesicle or bladder which functions as a float and helps in swimming, the pneumatophore is an inverted medusoid devoid of mesogloea, but its walls are muscular and it has gas glands. It shows great variation in its structure and size in different siphonophores.
2. Phyllozooids are leaf-like bracts or thick and gelatinous medusoids having a canal of the enteron. They are protective and shield some zooids of a colony.
3. Nectocalyces or nectophores are bell-shaped medusoids with a velum, radial canals and circular canal, they have no mouth, manubrium, tentacles or sense organs, A nectocalyx is muscular and brings about locomotion of the colony by swimming. It is also referred to as nectophore or nectozooid.
4. Gonophores occur singly or in clusters on blastostyles, they are degenerate medusae with no mouth, tentacles or sense organs, but they have a velum, canals and a manubrium which bears gonads. Gonophores are dioecious and produce germ cells for reproduction.
Origin of Polymorphism:
A number of theories are given to illustrate the origin of polymorphism in Coelenterata; some of these theories are as follows:
1. Poly-organ Theory:
This theory was proposed by Huxley, Eschscholtz and Metschnikoff.
According to this theory, a polymorphic colony is supposed to be a single medusoid zooid; its various components are regarded to be the modified organs of this medusoid zooid. The various parts of the zooid, i.e., manubrium, tentacles, umbrella, etc., multiply independently from one another and they have assumed different forms to perform different functions.
2. Poly-person Theory:
This theory was proposed by Leuckart, Vogt and Gegenbaur. According to this theory, a polymorphic coelenterate is supposed to be a colonial form in which various types of zooids have been aggregated to perform different functions. All the zooids of the colony are either polyps or medusae or both but the primitive zooid of the colony is of polyp type.
3. Haeckel, Balfour and Sedgwick have agreed that poly-person theory is more correct but they are of the view that the primitive zooid of the colony is probably medusoid which have produced other medusae by asexual budding. Thus, the zooids of the colony are nothing more than the organs of medusiform zooid which have shifted their attachment.
4. Recently, Moser has revived the poly-organ theory. According to him, the various zooids of the colony are organs which have not yet attained the grade of polymorphic form. Thus, the siphonophores are the most primitive existing coelenterates. This theory has not been recognised in general because it altogether denies the colonial nature of siphonophores.
Polymorphism and Alternation of Generations:
Polymorphism is, no doubt, a phenomenon of division of labour, i.e., different functions are attributed to different individuals of the polymorphic form, rather than to the parts or organs of one individual or zooid. All the members of the colony whether polypoids or medusoids, are formed from the coenosarc. Polymorphism is associated with the life cycles of Coelenterata.
In monomorphic forms, like Hydra, the polyp reproduces both asexually and sexually, this condition also applies to Anthozoa. The life cycle remains very simple; it may be represented as polyp-egg-polyp. With the origin of polymorphism, the reproductive powers of the organisms are divided among the different zooids of the colony.
In these cases the polyps reproduce asexually to give rise to medusoid forms (i.e., gonophores) which reproduce sexually to form polyp. The life cycle, thus, becomes complicated and may be represented as polyp-medusa-egg-planulapolyp. Therefore, the so called alternation of generations or metagenesis comes into existence where, in fact, asexual polypoid generation alternates sexual medusoid generation.
Significance of Polymorphism:
The polymorphism is essentially a phenomenon of division of labour in which different functions are performed by the different members or zooids of the colony, viz., polyps are related to feeding and asexual reproduction, while medusae are related to sexual reproduction and so on.
7. Corals of Coelenterata:
Corals are animals belonging to the phylum Coelenterata. These are solitary or colonial polypoid forms and live in a skeleton of calcium carbonate secreted by themselves. Some of them grow into huge mounds, while others are large and branched colonies. However, most of the corals belong to the class Anthozoa and few others to the class Hydrozoa.
The hydrozoan corals belong to order Milleporina, e.g., Millepora and order Stylasterina, e.g., Stylaster. These are colonial forms and secrete massive exoskeletons. Within the exoskeletons, two types of polypoid forms, the gastrozooids and branched dactylozooids are found.
In fact, the epidermis of these polypoid forms is modified and referred to as the calicoblast layer which secretes the calcareous exoskeleton. These corals are found with other corals forming huge mounds.
The anthozoan corals belong to subclasses Octocorallia and Hexacorallia both. The Octocorallia corals belong to order Stolonifera, e.g., Tubipora; order Alcyonacea, e.g., Alcyonium; order Coenothecalia, e.g., Heliopora and order Gorgonacea including sea fans like Gorgonia and Corallium.
The Hexacorallia corals belong to the order Madreporaria, e.g., Astraea, Fungia, Madrepora and Meandrina, etc.; and to the order Antipatharia, e.g., Antipathes.
Among Octocorallia, Stolonifera includes colonial corals like Tubipora or organ pipe coral. In it, the skeleton is made of fused calcareous spicules which form vertical parallel tubes connected together by platforms. The skeleton is tinged red by iron salts and the polyps lie in the tubes partly projecting above.
It is widely distributed on coral reefs. The Alcyonacea includes soft corals like Alcyonium. In it the coral is a colony of polyps having an endoskeleton of separate caleareous spicules embedded in its massive mesogloea.
The Coenothecalia includes a single genus of blue coral, the Heliopora. In it the coral is a colony of polyps which secrete calcareous spicules to form a massive skeleton called corallium. The Gorgonacea includes horny corals, the sea fans like Gorgonia. In it the coral is a branched colony of polyps which secrete a horny proteinaceous material with calcareous spicules around the polyps.
Among Hexacorallia, Antipatharia includes black corals like Antipathes. In it the colony is tree-like and its skeleton consists of branched chitinoid axis. The Madreporaria includes stony corals or true corals like mushroom coral (Fungia), star coral (Astraea) and brain coral (Meandrina or Meandra). Among these some are solitary, while most of them are colonial forms and the principal builders of the coral reefs.
The stony corals, in general, have polyps which are very similar in structure to sea anemones with tentacles and mesenteries in multiples of six, but they are different in having no siphonoglyphs and in being usually colonial. They have an ectodermal external skeleton of calcium carbonate. The exoskeleton of a polyp is called a corallite, and the exoskeleton of the colony is a corallum.
The epidermis of polyp secretes a basal disc of calcium carbonate, then walls of a cup called theca which immovably encloses the polyp, in the theca are radiating septa like mesenteries of an anemone, the septa bear nematocyst. In the centre of the theca is a vertical rod called columella to which the primary septa are fused.
All these form the exoskeleton of a polyp, the polyp fills the corallite and partly projects above. In a colony thousands of polyp form their corallites which are fused together to form a corallum, but all the polyps in a corallum are joined together by lateral connections. A coral colony increases in size by budding new polyps along the margin of the colony. Feeding in corals occurs only at night.
The corals, in general, have diverse shapes and sizes, some are solitary and have large polyps, but the majority are colonial with very small polyps. Some corals are used as ornaments and jewellery. Coral colonies are most abundant in tropical seas though some occur in arctic and temperate seas, but most of them flourish at a temperature above 22°C.