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Phylum Coelenterata (Cnidaria) Definition, Classification, Characteristics

The Coelenterata can be described as a diploblastic metazoa that has a the tissue grade having one nematocyst, and one gastrovascular cavity,...

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This article writter by MN Editors on November 20, 2021

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Phylum Coelenterata (Cnidaria) Definition, Classification, Characteristics
Phylum Coelenterata (Cnidaria) Definition, Classification, Characteristics

Definition of Phylum Coelenterata (Cnidaria) 

The Coelenterata can be described as a diploblastic metazoa that has a the tissue grade having one nematocyst, and one gastrovascular cavity, or coelenteron.

Characteristics of Phylum Coelenterata (Cnidaria) 

  • They are marine, mostly freshwater, with a few exceptions such as the Hydra.
  • They are multicellular, with a the ability to organize tissues.
  • They are conical or solitary. They are free-swimming or sedentary.
  • Individuals are biradially or radially in a symmetrical fashion around an oral-aboral axis that runs longitudinally.
  • Organization of the body of cells of the tissue grade. Cells are scattered and specialized for various functions. Certain cells are tissues, such as nerve nets or nerve tissues.
  • Exoskeleton Chitinous (perisarc) as well as calcareous(corals).
  • They are animals that have two cellular layers -outer an epidermis, and an internal gastrodermis. There is an acellular mesoglea of gelatinous in between.
  • Animals that are coelomate don’t pose an additional body cavity, the coelom.
  • The tentacles are short and slim, and they surround the mouth in two or three whorls.
  • The tentacles are stocked with nematocysts. They are used to capture food particles, intake, for adhesion, as well as for defense.
  • Two kinds of people exist in the form of attached sessile and asexual Zooid (polyps) and sexual zooid that is free and free of swimming (medusae). Certain species are notable due to different forms of polymorphism.
  • Carnivores are generally carnivores. digestion is extracellular and intracellular.
  • There is no any.
  • Coelom as well as respiratory, circulatory and excretory system lacking.
  • The primitive nervous system consists of a nerve network that is diffuse. Central nervous system is absent.
  • The muscular system is comprised of circular and longitudinal fibers created by epithelia-muscle as well as endothelial muscle cells.
  • A single cavitythat is lined with gastrodermisis known as a coelenteron or gastrovascular that opens the mouth.
  • Sensory organs in the form of ocelli and statocysts.
  • Reproduction can be achieved through sexual or asexual methods.
  • The process of reproduction is asexual, and involves sexual reproduction and budding through the development of gametes.
  • The larva is free-swimming larva of the planula.
  • Life history shows the phenomenon of alternation of generations or metagenesis where the asexual, sessile generation alters with sexual medusoid free-swimming generation.

Classification of Phylum Coelenterata (Cnidaria) 

The phylum Coelenterata contains more than 11,000 species of which half are extinct. The classification used here is provided by Hyman, L. H., (1940). Based on Hyman phylum Coelenterata is classified into three classes.

Class 1. Hydrozoa (Gr., hydra=water +zoios=animal)

  • Marine or freshwater. Conical or solitary. Free-swimming or sessile.
  • Show tetramerous and polymerous Radial and radial.
  • The body wall is comprised of ectoderm with outer and inner endoderm that is separated by mesoglea that is non-cellular.
  • Gastrovascular cavity with no stomodaeum septas, or even nematocysts that bear gastric filament.
  • The horny or skeleton structure is a horny perisarc found in a few forms, while coenosarc produces the skeleton of calcium carbonate creating an enormous stony structure, or coral in various forms.
  • They show polymorphism. They are two species of zooids, which are the asexual polyp and sexual medusa.
  • Polyp with no stomodaeum or septa (mesentery).
  • Medusa with real Velum (Craspedote).
  • Mesoglea non-cellular.
  • Many of them show the alternation of generations.
  • Epidermal. Sex cells shed right on the exterior.
  • Cleavage is holoblastic and embryo ciliated in the planula.

Order 1. Hydroida

  • Conical or solitary.
  • Polypoid stage dominant.
  • Medusae are very short-lived or not present.
  • The sense organs of medusae are the ocelli and statocysts, which are of ectodermal origin.

Suborder 1. Anthomedusae or Athecata

  • Conical or solitary.
  • Polyps and blastostyles are athecate , i.e. perisarcs are not producing hydrothecae, and gonothecae.
  • Medusae are bell-shaped, tall plants with gonads hanging from the manubrium with a heavily arched umbrella.
  • Medusae are ocelli or eyespots at the base of tentacles.
  • Statocysts are absent.
  • Examples: Hydra, Ceratella, Tubularia, Clava, Eudendrium.

Suborder 2. Leptomedusae or Thecata

  • Conical Hydrozoa.
  • Polyps are contained in medusae and hydrothecae. Both are covered by gonothecae.
  • Medusae that are free of charge can be flattened bowl – or saucer-shaped and bear Gonadals along the radial canal.
  • Medusa with gonads located on canals radiatal.
  • Medusae typically carry statocysts.
  • Eyespots and ocelli are not present.
  • Examples: Obelia, Sertularia, Plumularia, Aglaophenia.

Order 2. Milleporina

  • Conical coral-like Hydrozoa with no perisarc.
  • The massive calcareous skeleton made by ectoderm, which has pores where polyps grow out.
  • Colony are composed of two kinds of zooids. The gastrozooid as well as the dactylozooid.
  • Gastrozooids (nutritive Zooids) are small and come with tentacles and mouth.
  • Dactylozooids are hollow, elongated and slender, with tentacles but with no mouth.
  • Medusae appear in small chambers before becoming free with no the mouth and radial canals and tentacles.
  • Example: Millepora.

Order 3. Stylasterina

  • Coral-like colonial colonies Hydrozoa colony has two kinds of zooids, namely the gastrozooids and the dactylozooids.
  • Dactylozooids are tiny solid with no tentacles.
  • Gastrozooids come with a cup that has a an angled spine.
  • Gonophores are reduced to the sporosacs. Medusae not for sale.
  • Larva can be liberated by planula.
  • Example: Stylaster.

Order 4. Trachylina

  • The stage of polypoid is reduced or not present.
  • Medusae are huge and dominant, they are free-swimming and can develop from fertilized eggs.
  • Statocysts and Marginal Sense Organs that contain endodermal statoliths.

Suborder 1. Trachymedusae

  • Tentacles placed above bell’s margin.
  • The umbrella’s edge is smooth.
  • Manubriums are long.
  • Gonads develop in radial canals.
  • Example: Geryonia.

Suborder 2. Narcomedusae

  • Tentacles form between the bell’s edge and the vertex of exumbrella.
  • The manubrium is a short.
  • Gonads are found in the manubrium or the floor of your stomach.
  • Examples: Cunina, Solmaris.

Order 5. Siphonophora

  • They are free-swimming, polymorphic, and floating colonial Hydrozoa.
  • The colony is comprised of various kinds of medusoid and polypod individuals that are attached to stems or disc.
  • Polyps that do not have oral tentacles.
  • Medusae insufficient and seldom freed.

Suborder 1. Calycophora

  • The top in the colonies is equipped by one of the bells for swimming (nectophores).
  • Apical float or Pneumatophore missing.
  • Examples: Diphyes, Praya, Abyla.

Suborder 2. Physophorida

  • The upper ed of the colony is an enormous gas-filled floating (pneumatophore).
  • Examples: Physalia, Halistemma, Stephalia.

Class 2. Scyphozoa (Gr., skyphos=cup +zoios=animal)

  • It is comprised of jelly-fish that are large or medusae true medusae, which are only marine.
  • Medusae are huge, bell or umbrella-shaped, but without velum, and are free-swimming or connected to the aboral stalk.
  • Polyp stage is diminished or absent.
  • The tentaculocysts of the margin are with statoliths of the endoderm.
  • Gastrovascular cavity that contains gastric pouches as well as gastric filaments that end in the endodermal region. There is no Stomodaeum.
  • Mesoglea is a large, gelatinous, filled with cells and fibers.
  • Gastrodermal. Sex cells are released from the digestive tract.

Order 1. Stauromedusae or Lucernaridae

  • The body goblet or trumpet is shaped.
  • Sessile is attached to the aboral stalk.
  • Mouth Cruciform (four-cornered) with smaller lobes for the mouth and a manubrium that is short, quadrangular.
  • It is separated into the stomach’s central part and four pouches per-radial via those four septa interradial.
  • Gonads are bands of elongated length that appear on the septa’s faces.
  • There are no marginal sense organs, or tentaculocysts.
  • Fertilization external.
  • The larva is planula , but without cilia.
  • Examples: Lucernaria, Haliclystus.

Order 2. Cubomedusae or Carybdeida

  • Body cubical that has four sides that are flattened.
  • Free-swimming Scyphozoa is found in shallow and warm waters of subtropical and tropical regions.
  • Four hollow inter-radial tentacles that are borne along the edge of the subumbrella.
  • 4 per-radial tentaculocysts, or rhopalia, are present.
  • Each tentaculocyst comes with the lithocyst as well as one or more Ocelli.
  • It is also cruciform in its mouth. gastric pouches are found.
  • Leaf-like gonads.
  • Examples: Charybdaea, Tamoya.

Order 3. Coronate

  • Conical body, separated by an extensive circular coronary groove.
  • The free-swimming scyphomedusae are found deep waters of the ocean.
  • It is separated by an oblique groove (horizontal furrow) into an upper cone and a lower.
  • The crown is composed of the pedal lobes, a pedalia.
  • The pedalia are a solid set of tentacles.
  • The bell’s edge is cut into lappets that alternate with pedalia.
  • Cruciform mouth.
  • Between 4 and 16 tentaculocysts are present.
  • Examples: Pericolpa, Periphylla.

Order 4. Semaeostomeae

  • Medusae are the most common free-swimming species in the waters off the coast across all oceans.
  • The umbrella is flatsaucer, flat or bowl-shaped.
  • A square-shaped mouth that extends into four long oral arms.
  • The umbrella’s edge is surrounded by hollow tentacles.
  • 8 tentaculocysts are present.
  • The gastric pouches as well as the filaments are missing.
  • Examples: Aurelia, Cynaea.

Order 5. Rhizostomae

  • The body usually is hemisphere with no any tentacles at the margins.
  • Scyphozoa that swims freely in the shallow waters of subtropical and tropical oceans.
  • The umbrella can be saucer- or bowl-shaped, or flattened, or concave at the top.
  • It is enclosed by eight oral arms that have funnel-shaped mouths at their edges.
  • Typically eight plus tentaculocysts.
  • The pits in four subgenitals are usually found.
  • Examples: Rhizostoma or Pilema, Cassiopeia.

Class 3. Anthozoa (Gr., anthos= flower+ zoios= animal)

  • Only marine. Conical or singular.
  • Only polypoid.
  • No medusoid stage.
  • The body is generally cylindrical and has hexamerous, Octamerous or polymerous biradial or radobilateral.
  • The end of the oral body expands radially to form an oral disc with tentacles that are hollow and surround the mouth at the center.
  • The stomodaeum can be found, typically with one or more grooves ciliated, known as siphonoglyphs.
  • The gastrovascular cavity is subdivided into 8 or more septas or mesenteries.
  • Mesenteries are characterized by nematocysts that form their edges that are free.
  • Mesoglea robust and contains fibrous connective tissue as well as an amoeboid cells.
  • Skeleton, whether internal or external.
  • The exoskeleton is made of calcium carbonate, which is often an enormous coral.
  • The nervous system takes shape of typical nerve network with no nerve system central to it.
  • Endodermal gonads are a form of gonads that mesenteries.
  • The sexually ripe products are released into coelenteron.
  • External fertilization.
  • The fertilized egg evolves into a planula-like larva which , after a short life , is able to settle down and grow in to an adult.

Subclass 1. Alcyonaria or Octocorallia

  • Only colonial.
  • Polyps are short or long cylindrical structures that are terminated orally into circular disc that is flat and has the shape of an oval, or with an elongated or oval-shaped mouth in the middle.
  • Polyp with 8 tentacles with pinnate as well as 8 septa.
  • 8 mesenteries that are complete are in place.
  • Single ventral siphonoglyphs present.
  • Endoskeleton is the result of mesogleal cells that are composed of calcareous spicules, which is the case in nature or horny.
  • Polyps can be dimorphic in a certain form.

Order 1. Stolonifera

  • People who live in shallow waters that are found in tropical or temperate regions.
  • Polyps that develop in isolation from a mat with a creeping effect or Stolon.
  • Skeleton made of calcareous tubes distinct calcareous spicules, or none at all.
  • Examples: Tubipora, Clavularia.

Order 2. Telestacea

  • The colony is composed of stems that are simple or branched that arise from a creeping base.
  • Each stem is an extended polyp that has two polyps.
  • Skeleton is composed of calcareous spikes.
  • Example: Telesto.

Order 3. Alcyonacea

  • Colony mushroom-shaped, or branched into sharp, stout processes.
  • The lower parts of the polyp merged into coenchyma or a fleshy mass with the oral end protruding.
  • Polyps are dimorphic in a few form-bearing autozooids and siphonozooids.
  • Skeleton is made up of distinct clacerous spicules that are not axial.
  • Examples of soft corals. Alcyonium, Xenia.

Order 4. Coenothecalia

  • Skeleton is a massively formed of calcareous crystalline fibers made of calcium carbonate but not of spicules that have been fused.
  • Polyp embedded, and joined by solennial tubes.
  • Blue corals are commonly located on coral reefs of the Indo-Pacific.
  • Example: Heliopora (blue coral).

Order 5. Gorgonacae

  • The colony, which is typically plant-like comprises the main stem that is derived out of the plate, or the stolon tuft and several branches that carry polyp.
  • A x-ray skeleton made of horn-like gorgonin. It can be separated or fused clacerous spicules or both.
  • Also known as sea fan or sea feathers. They also include sea whips.
  • It is found in subtropical and tropical shores.
  • Examples: Gorgonia, Corallium.

Order 6. Pennatulacea

  • Colony that is elongated and split into a stalk that is proximal or the peduncle, and a the distal rachis.
  • The lower part(peduncle) is embedded in sand and mud.
  • The upper portion (rachis)consists of a long axial polyp that has branches that are lateral and bear dimorphic polyp.
  • Main stems are supported by a calcium-rich, or horny the skeleton.
  • Examples: Pennatula, Renilla, Cavernularia, Pteroides.

Subclass 2. Zoantharia or Hexacorallia

  • Colonial or solitary.
  • Marine form.
  • Tentacles that are unbranched and simple many arranged in multiples of five and six, but never 8.
  • Mesenteries vary in groups of 5 or 6. may be completely or in part.
  • Gullet is usually accompanied by two siphonoglyphs.
  • Endoskeleton, when calcareous and formed from ectoderm.
  • Monomorphic polyps are usually monomorphic.

Order 1. Actiniaria

  • Colonial or solitary.
  • Simple and often big-sized.
  • No skeleton.
  • Muscles in the body, typically associated with an aboral pedal disc.
  • Tentacles and mesenteries are abundant.
  • Siphonoglyph One of or many.
  • Examples: Actinia, Metridium, Adamsia, Edwardsia.

Order 2. Madreporaria

  • Rarely, or usually colonial.
  • The exoskeleton is extremely hard and compact, and can be massively and calcareous.
  • Small polyps, which live inside cup-like cavities of the exoskeleton.
  • There is no siphonoglyph, and muscles are weak.
  • Examples: stony or true corals. Astraea (star coral), Fungia, Favia, Madrepora (staghorn coral), Meandrina (brain coral).

Order 3. Zoanthidea

  • The majority of colonial forms are solitary, although they can be solitary.
  • There is no pedal disc and skeleton however, the wall of the body is made up of calcareous bodies.
  • Most often epizoic.
  • Small polyps that are typically joined through basal Stolons.
  • Paired mesenteries. A mesentery pair consisting of one completed and one mesentery that is not complete.
  • There is only one ventral siphonoglyph.
  • Example: Zoanthus.

Order 4. Antipatharia

  • Tree-like and colonial.
  • It is found in the deep water within the oceans.
  • The lower part of the colony is usually comprised of a base plate that is used for attaching objects.
  • Tentacles, mesenteries and mesenteries are relatively few (6-24) in terms of numbers.
  • Skeleton as a chitinoid axis that is branched and which is derived from the ectoderm.
  • The axial skeleton contains polyps that are dioecious, but the colony could be hermaphrodite.
  • Two siphonoglyphs are present.
  • Examples include: Black corals. Antipathes.

Order 5. Ceriantharia

  • Anemone-like forms that live in vertically cylindrical spaces of the bottom of the sea.
  • There is no pedal disc or skeleton.
  • Body smooth , cylindrical and extended with the oral disc.
  • Tentacles that are simple, many with two whorls- marginal and oral.
  • Dorsal and single siphonoglyphs.
  • Mesenteries can be numerous, singular and full.
  • Examples: Cerianthus.
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