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In this article we will discuss about Cnidaria:- 1. Definition and Origin of Cnidaria 2. Habit and Habitat of Cnidaria 3. Characteristic Features 4. Classification.
Definition and Origin of Cnidaria:
Diploblastic metazoans with radial symmetry, tissues but rarely organs and a single central body cavity with mouth and tentacles but no anus, and the stinging cells or nematocysts, and with ciliated planula larva, called Cnidaria.
Origin:
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Pre-Cambrian.
Habit and Habitat of Cnidaria:
Phylum Cnidaria constitutes the first metazoan proper having well-developed tissue grade of organization. This phylum is notable in the animal kingdom for exhibiting polymorphism. Most of them are marine and others are freshwater. They are mostly pelagic animals and are free-living. Instances of commeansalim are quite numerous amongst the actinozoans.
Parastisim, through very rare, is also not uncommon, Polypodium, one of the Anthomedusae, remains as a parasite in the ovary of a fish, called Sturegon. Cunina, one of the Narcomedusae, lives parasitically on Trachymedusae. Peachia, an actinoza, leads parasitic life in the larval condition in the radial canals of Scyphomedusae.
Characteristic Features of Cnidaria:
1. Cnidarians are aquatic and radially symmetrical animals.
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2. Body represents tissue grade of organization but no organs or rarely organs.
3. Embryonic cnidaria are diploblastic in nature with an outer layer of ectoderm (develops epidermis in adult) and an inner layer of endoderm (develops gastrodermis in adult) separated by a non-cellular jelly-like layer called mesoglea or partly cellular mesenchyme derived primarily from ectoderm.
4. Presence of a gut cavity lined by the endoderm, the coelenteron or gastro- vascular cavity with a single opening to the exterior, mouth encircled by tentacles, and the anus being absent. The mouth also functions as anus.
5. Presence of highly specialized intracellular structures—the cnidoblasts (or nematoblasts) which secret cnidae. The cnidae or nematocysts or stinging threads are a unique to the members of the phylum, serve for defence, offence, food capture and adhesion.
6. Digestion is initially extracellular, then intracellular.
7. Nervous system represents as simple nerve nets at the base of the epidermal and gastrodermal layers with scattered and nonpolar nerve cells.
8. No respiratory, circulatory or excretory organs.
9. Cnidarians exist in two forms—polyp (representing asexual generation) and medusa (representing sexual generation). The polyp is tubular and usually remains fixed to the aboral end and the free end of the polyp possesses a conical elevation, called hypostome or manubrium.
The free- swimming medusa is bell-shaped or saucer-shaped with dorsal convex exumbrellar surface and a ventral concave subumbrellar surface.
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10. Presence of asexual and sexual reproduction. Asexual reproduction by budding and sexual reproduction by the formation of ova and sperm.
11. Cleavage may be complete or incomplete and usually radial.
12. In small-sized eggs, the division is complete and a hollow blastula (coeloblastula) is formed and in large- sized eggs, the division is incomplete and a solid blastula (stereoblastula) is formed due to the remaining of the yolk mass in the centre of blastula.
13. Planula larva represents a solid gastrula stage (stereogastrula) consisting of an outer ciliated ectoderm and inner endodermal cells.
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14. The life cycle includes the phenomenon either of alternation of generations or metagenesis.
Classification with Characters:
Class 1. Hydrozoa (Hydras, sea furs; about 3000 species) [Gk. Hydor= water + zoon = animal]
The class Hydrozoa (Figs. 12.27, 12.28 and 12.29) is characterised by the possession of the following characteristic features:
Features:
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1. Exclusively polypoid or exclusively medusoid or both forms in the life cycle.
2. Coelenteron undivided and without stomodaeum.
3. Symmetry polymerous or tetramerous.
4. Nematocysts are confined to the epidermis.
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5. The gastro-dermal tissue lacks nematocysts.
6. Mosogloea non-cellular.
7. Medusa with a true velum.
8. Metagenesis distinct.
9. Reproductive cells usually epidermal in origin and discharged to the exterior directly.
10. Mostly colonial and marine; a few solitary and freshwater.
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Order 1. Trachylina:
1. Medusoid generation well-represented and with or without a polypoid stage.
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2. Sense organs are statocysts or tentaculocysts and consist partly of endoderm.
3. Medusa develops directly from an actinula larva.
The order includes two suborders:
(1) Trachymedusae and
(2) Narcomedusae.
1. Suborder Trachymedusae:
(a) The margin of the bell is smooth.
(b) The gonads are borne on the radial canals.
The examples are Rhopalonema, Aglaura, Geryonia, Liriope, Glossocodon.
2. Suborder Narcomedusae:
(a) The margin of the bell is scalloped by tentacles.
(b) The gonads are present on the floor of stomach or in manubrium.
The examples are Cunina, Cunoctantha, Aegina, Hydroctena, Solmaris.
Order 2. Hydroida:
1. Polypoid stage well-represented and usually fixed.
2. Sense organs exclusively ectodermal in origin.
3. Solitary or colonial forms. The order includes majority members of hydrozoans.
Suborder Limnomedusae:
1. Small, solitary polyps and free medusae.
2. Mostly freshwater forms, some groups in lakes of Africa and India; a few species marine.
Examples: Craspedacusta (widespread freshwater jellyfish), Pochella (marine), Gonionemus (marine).
Suborder Anthomedusae (Athecata or Gymnoblastea):
1. Gastrozooids without hydrothecae (Athecata).
2. Naked blastostyles.
3. Free medusae tall and bell-like.
4. Sense organs in the form of ocelli but without statocysts.
5. Gonads borne on the manubrium.
6. Solitary or colonial hydrozoa.
7. Inhabitants of freshwater or marine.
Examples:
Protohydra, Hydra, Sarsia, Corynitis, Steenstrupia, Corymorpha, Bougainvillea, Tubularia, Clavia, Corydylophora, Hydractinia, Brachiocerianthus, Millepora (the hydrocorals).
Suborder Leptomedusae (Thecata or Calyptoblastea):
1. Gastrozooids with hydrothecae (Thecata).
2. Blastostyles covered by gonothecae.
3. Hydranth surrounded by a skeleton (thecate).
4. Flattened or saucer-shaped free medusae.
5. Sense organs in the form of statocysts.
6. Gonads borne on the radial canals.
7. Colonial hydrozoa.
8. Marine.
Examples:
Laodicea, Staurophora, Obelia, Clytia, Campanularia, Sertularia, Plumularia, Ptychogena, Melicertum, Polyorchis.
Suborder Chondrophora:
1. Polymorphic and polypoid colonies.
2. Single large polyp with gas filled, chitinous, multichambered, disc-like float or pneumatophore with an oblique sail may or may not be present.
3. Gonozooids bear medusiform gonophores.
4. Marine and pelagic.
Examples:
Porpita (Blue button), Velella (By-the-wind sailor), Porpema.
Remark:
Marshall and Williams (1972), and Pechenik (2000) have created a separate order Chondrophora for Porpita and Velella. Hyman (1940) included Porpita and Velella under the suborder Physophorida and order Siphonophora.
Order 3. Actinulida:
1. Very small, measuring less than 1-5 mm long.
2. No free living medusa.
3. Polyps somewhat resemble the actinula larva of the trachylinids.
4. Epidermal cells ciliated.
5. Marine and solitary.
6. All individuals are interstitial inhabitants, i.e., live in the spaces between sand grains.
Example:
Halammohydra, Otohydra.
Order 4. Siphonophora:
1. Colonial forms exhibit extreme polymorphism of both the polyp and medusa.
2. Polyps without oral tentacles.
3. Gonophores do not develop into complete medusae.
4. Colonies with gas-filled floats (Pneumatophore), or large swimming bells in some.
5. Marine, pelagic and mainly found in the tropical seas.
Examples:
Diphyes, Praya (Pneumatophore absent), Agalma, Physalia (Portuguese man-of- war), Nectalia, Stephalia, Halistemma, Rhizophysa.
Order 5. Stylasterina:
1. Dactylozooids small, solid and without tentacles.
2. Gonophores reduced to sporosacs.
3. Hydrozoans secrect calcium carbonate skeletons.
4. Gastrozooids have a cup with pointed spine.
5. Marine and colonial.
Examples:
Stylaster, Allopora, Astylus, Cryptohelia, Distichopora.
Class 2. Scyphozoa:
(True jellyfish, About 200 species) [Gk. Skyphos = cup; zoon = animal]
Features:
1. Adults exhibit only the dominant medusoid stage and the polypoid stage is very insignificant. The body of medusae is bell or umbrella-shaped.
2. Polyp represented by Scyphistoma (Polyp-like larva of a jellyfish) and Hydratuba (Hydra-like larva).
3. Gastrovascular cavity contains mesenteries except Rhizostomeae.
4. Symmetry teramerous (Having four parts).
5. Endodermal gastric tentacles present.
6. Mesogloea cellular and thick.
7. Velum absent.
8. Sense organs usually in the form of tentaculocysts.
9. Sexes generally separate and the species is said to be gonochoristic.
10. Gonads endodermal in origin and are closely associated with the gastric pouches.
11. Each polyp gives rise to many medusae by strobilation.
12. Marine and solitary. They are found in cold and tropical oceans.
Order 1. Stauromedusae or Lucernariida:
1. Umbrella conical or vase-shaped and remains attached to the substratum by aboral peduncle.
2. Tentacles usually absent.
3. Tentaculocysts absent.
4. The planulae are non-ciliated and creeping animals.
5. Inhabitants of cold littoral waters.
Examples:
Lucernaria, Craterolophus, Haliclystus.
Order 2. Coronatae:
1. Medusae free-swimming
2. Umbrella divided by a deep circular groove or constriction, the coronal groove.
3. Tentaculocysts four to sixteen.
4. Many deep sea forms.
Examples:
Periphylla, Stephanoscyphus, Pericolpa, Nausithoe, Atolla.
Order 3. Semaeostomeae:
1. Umbrella disc-shaped or saucer- shaped and usually eight tentaculocysts.
2. Margin of the umbrella fringed with hollow tentacles.
3. Mouth square and manubrium divided into 4 oral arms.
4. Gastric pouches and filaments absent.
5. Free-swimming medusae inhabiting the coastal waters of all oceans.
Examples:
Aurelia (Moon jelly), Cyanea (Lion’s mane jellyfish), Chrysaora (Sea nettle), Pelagia.
Order 4. Rhizostomeae:
1. Bell of medusa is devoid of tentacles.
2. Original mouth is obliterated through the fusion of the oral arms except in Stomolophus.
3. Many small mouths and canals present in the oral arms.
4. 8 or more tentaculocysts present.
5. Mostly in shallow waters of tropical and subtropical seas.
Examples:
Pilema, Cassiopea, Rhizostoma, Stomolophus, Acromitus.
Class 3. Cubozoa (Sea wasps; About 20 species) [Gk. Cubo = cube; zoon = animal]:
Features:
1. Small, medusoid cnidarians with a highly transparent cuboidal swimming bell.
2. Bell margin simple and bearing a velum and four tentacles or four clusters of tentacles.
3. Tetramerous symmetry.
4. Four rhopalia (marginal sense organs) at the corners.
5. Medusa stage dominant.
6. Presence of a well-developed nervous system and complex eyes.
7. Presence of virulent nematocysts in the tentacles.
8. Attached polypoid stage develops from the planula larva.
9. Marine; mainly found in the Pacific Ocean.
Examples:
Charybdaea, Chironex, Tripedalia, Chiropsalmus.
Chironex fleckeri is found in the coastal waters of Australia. It is considered one of the most deadly of all marine animals. Death takes place within 3 to 20 min after stinging.
Remark:
Formerly Cubozoa was considered an order under the class Scyphozoa but due to lack of a notched bell margin and polyp stage without strobilation, the individuals should be placed within a separate class.
Class 4. Anthozoa or Actinozoa (Sea anemones, Corals; about 6000 Species) [Gk. Anthos = flower; zoon = an animal]
Features:
1. Cnidarians represent only the polypoid form in the life cycle.
2. Medusa stage absent.
3. Body cylindrical with symmetry hexamerous or octomerous.
4. Stomodaeum (foregut) strongly developed and possess siphonoglyphs (ciliated grooves in the stomodaeum).
5. Coelenteron (= gastrovascular cavity) partitioned by mesenteries.
6. Mesoglea cellular with fibrous connective tissue and amoeboid cells.
7. Some cnidocytes are gastrodermal.
8. Gonads are gastrodermal in origin and develop in the mesenteries.
9. Gametes are produced directly by the polyp.
10. The fertilized egg develops into a planula larva that metamorphoses to form the polyp.
11. Solitary or colonial; marine or brackish water.
It includes two subclasses:
(i) Octocorallia and
(ii) Hexacorallia.
(i) Subclass Octocorallia or Alcyonaria: [L. Octo – eight, Gk. korallium = coral]:
1. Polyps always possess pinnately branched tentacles in multiples of eight.
2. 8 complete mesenteries present.
3. One ventral siphonoglyph only present.
4. Colonial marine forms with endoskeleton.
Order 1. Stolonifera:
1. Polyps not fused and connected by basal stolons.
2. No coenenchymal mass.
3. Skeleton of calcareous tubes of separate or fused calcareous spicules.
Example:
Tubipora (Organ-pipe coral), Clavularia.
Order 2. Telestacea:
1. Colony of long axial polyps.
2. Many lateral polyps on simple or branched stems.
Example:
Telesto.
Order 3. Alcyonacea:
1. Polyps united in the proximal parts to form the fleshy mass.
2. Skeleton of separate calcareous spicules.
3. Soft corals without a rigid skeleton.
4. Marine, specially in the tropical seas.
Examples:
Alcyonium (Dead-men’s fingers), Sinularia, Sarcophyton.
Order 4. Helioporacea:
1. Presence of a massive broad-lobed blue calcareous skeleton.
2. Perforated skeleton.
Examples:
Heliopora, Epiphaxum (Blue Corals).
Order 5. Gorgonacea (Gorgonian corals):
1. Corals are upright tree-like in appearance.
2. A calcareous or horny skeleton of ectodermal origin present.
3. Separate or fused calcareous spicules present.
4. Siphonoglyph absent.
5. Horny or gorgonial corals.
Examples:
Gorgonia (Sea fan), Leptogorgia (Sea whip), Corallium (Red corals), Muricea (Sea rod), Isis, Parisis, Psammogorgia, Swiftia.
Order 6. Pennatulacea (Sea Pens, Sea Pansies):
1. Colony consists of a long fleshy axial polyp, called rachis, with many lateral polyps
2. Lower part of the axial polyp called stalk or peduncle is devoid of polyps.
3. Polyps dimorphic.
4. Skeleton with calcareous spicules.
Examples:
Pennatula (Sea pen), Pteroides, Renilla (Sea pansy), Balticina, Cavernularia, Virgularia, Veretillum, Umbellula.
(ii) Subclass Hexacorallia (= Zoantharia) [Gk. Hex = six, korallium = coral]:
1. Solitary or colonial marine forms.
2. Tentacles and mesenteries generally numerous and in the multiple of 5 or 6.
3. Tentacles hollow and un-branched.
4. One pair of siphonoglyphs usually present in the pharynx.
Order 1. Zoanthidea (= Zoanthinaria):
1. No skeleton and padal disc.
2. One ventral siphonoglyph.
3. Unbranched tentacles.
4. Solitary or colonial.
Examples:
Zoanthus, Palythoa, Parazoanthus.
Order 2. Actiniaria:
1. No skeleton.
2. Numerous tentacles and mesenteries usually in multiples of six.
3. Usually two siphonoglyphs.
4. Solitary anthozoans.
5. Commonly called sea anemones.
Examples:
Adamsia, Tealia, Actinia, Minyas, Cryptodendrum, Urticina, Peachia, Metapeachia, Halianthus, Pelocoetes, Edwardsia, Stichodactyh, Phytocoetes, Phytocoeteopis, Anemonia, Paracondylactis, Metridium, Halcampa, Heteractis, Macrodactya, Sphenopus, Gyrostoma, Stomphia.
Order 3. Scleractinia (= Madreporaria):
1. Compact, massive calcareous skeleton.
2. No siphonoglyphs.
3. Mostly colonial.
4. Stony corals.
Examples:
Acropora, Madrepora (Stag- horn coral), Fungia (Mushroom coral), Flabellum, Porites, Oculina (Ivory coral), Astraea, Favia, Meandrina (Brain coral), Diploria (Brain coral), Montipora, Pachyseris, Psammocora, Siderastrea, Favites, Platygyra, Goniastrea, Helipora, Echinopora, Pavona, Galaxea (Star coral).
Order 4. Corallimorpharia:
1. Absence of skeletons.
2. Tentacles often in radiating rows.
3. Mostly solitary.
Examples:
Discosoma, Ricorda.
Order 5. Ceriantharia:
1. Animals living in secreted tubes buried in sand or mud.
2. No skeleton and pedal disc.
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3. Tentacles many, arranged in two whorls.
4. Single dorsal siphonoglyph.
5. Numerous mesentaries all complete.
6. Solitary forms.
Examples:
Arachnanthus, Cerianthus, Pachycerianthus, Ceriantheopsis.
Order 6. Antipatharia:
1. Colonial, tree-like forms.
2. Skeleton in the form of a branched, black chitinoid axis bearing thorns.
3. Tentacles fewer and mesenteries 6 to 24.
4. Two siphonoglyphs.
5. Black or thorny corals.
6. Mainly found in deep water of tropical seas.
Examples:
Antipathes (Black coral).
