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Revision Notes on Phylum Cnidaria and Ctenophora

Phylum Cnidaria (or Coelenterata)

(i) ‘Tissue grade’ eumetazoans with a radial symmetry.

(ii) The term “Coelenterata” signifies the presence of a single internal cavity called coelenteron, or gastrovascular cavity, combining functions of both digestive and body cavities. The term “Cnidaria” indicates the presence of stinging cells (Gr., knide = nittle or stinging cells).

(iii) Phylum coelenterata has the following salient features –

  • Coelenterates are multicellular organisms

  • They have tissue-grade of organization

  • The body is radially symmetrical. Radial symmetry is the symmetry of a wheel

  • All the members of this phylum are aquatic

  • They are solitary or colonial

  • Polyps and medusa occur in the life cycle.

  • The body wall is diploblastic.

  • Nematocysts or stinging cells are present.

  • Coelom is absent; hence coelenterates are acoelomate animals

  • A gastrovascular cavity or coelenteron is present.

  • Mouth is present; but anus is absent

  • Digestion is extracellular as well as intracellular

  • Respiratory, excretory and circulatory system are absent

  • Nervous system is diffuse-type, formed or nerve-nets.

  • Reproduction is by asexual and sexual methods

  • Development is indirect as there are one or two larval forms

  • Life history has alternation of generations or metagenesis.

Classification of coelenterata

Class 1 - Hydrozoa

  • Hydrozoa is solitary and fresh water or mostly colonial and marine.

  • They exhibit tetramerous or polymerous radial symmetry

  • Body wall consists of an outer ectoderm and an inner endoderm separated by a mesogloea.

  • Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament

  • Skeleton or horny structure is horny perisarc in some forms.

  • They exhibit polymorphism.

  • Many hydrozoa exhibit alternation of generation

  • Reproductive products of sex cells are usually ectodermal in origin and discharged externally.

  • Cleavage is holoblastic, embryo ciliated planula.

  • Examples : Hydra, Tubularia, Bougainvillea, Hydractinia, Eudendrium, Pennaria, Obelia, Sertularia, Plumularia

  • Physalia is commonly known as Portuguese man of war. Aurelia is commonly known as Jellyfish.

Class 2 - Scyphozoa

  • Scyphhozoa includes large jellyfishes or true medusae.

  • They are exclusively marine.

  • Medusae are large, bell or umbrella-shaped and without true velum.

  • Marginal sense organs are tentaculocysts

  • Polypoid generation is absent or represented by small polyp.

  • Gastrovascular system is without stomodaeum, with gastric filaments.

  • Mesogloea is usually cellular

  • Gonads are endodermal and the sex cells are discharged into the stomach.

  • Examples: Lucernaria, Haliclytus

Class 3 - Anthozoa

  • These are solitary or colonial exclusively marine forms

  • They are exclusively polypoid. Medusoid stage is altogether absent

  • Body is cylindrical with hexamerous, octomerous or polymerous biradial or radiobilateral symmetry

  • The stomodaeum is often provided with one or more ciliated grooves, the siphonoglyphs.

  • Gastrovascular cavity is divided into compartments by complete or incomplete septa or mesenteries.

  • Mesenteries bear nematocysts at their free edges

  • Mesogloea contains fibrous connective tissue and amoeboid cells.

Subclass 1 - Alcyonaria (Octocorallia)

  • These are colonial marine forms

  • Polyps are long or short cylinders

  • Polyps always bear eight pinnate, hollow tentacles

  • Eight complete mesenteries are present.

  • Single ventral siphonoglyph is present

  • Endoskeleton is the product of mesogloeal cells

  • Polyps are dimorphic in some forms.

  • Examples: Tubipora, Calvularia, Alcyonium, Xenia, Heliopora, Gorgonia, Corallium, etc.,

  • Tubipora is commonly known as organ pipe coral.

Subclass 2 - Zoantharia (Hexacorallia)

  • These are solitary or colonial marine forms

  • Tentacles simple, rarely branched, hollow cone shaped

  • Mesenteries are numerous arranged in the multiple of five or six

  • Two siphonoglyphs are commonly present

  • Endoskeleton when present is calcareous, derived from ectoderm

  • Polyps are usually monomorphic.

  • Examples: Actinia, Metridium, Adamsia, Edwardsia, Astraea, Fungia, Zoanthus, Antipathes

  • Metridium & Adamsia is commonly known as sea anemone.

Phylum Ctenophora

(i) Ctenophora is a small phylum. These animals exhibit the characters of Coelenterata and platyhelminthes.

(ii) Phylum Ctenophora shows the following salient features

  • All the ctenophores are marine.

  • They are solitary and pelagic.

  • They are transparent.

  • They have tissue-grade of organization.

  • They have biradial symmetry.

  • They are acoelomate animals.

  • They are non-segmented.

  • Their body-wall is diploblastic.

  • The mesogloea contains cells.

  • Nematocysts are absent.

  • Special adhesive cells called colloblasts are present in all ctenophores.

  • The gastrovascular system is well developed.

  • Two anal openings are present.

  • Skeletal system is absent.

  • Excretion and respiration are carried out by diffusion.

  • The nervous system is in the form of nerve net.

  • An aboral sense organ in present in the form of statocyst.

  • Cilia are used for locomation.

  • They are hermaphrodites.

  • Development is indirect. It includes a cydippid larva.

Classification of Ctenophora

Class 1 - Tentaculata

  • The body is simple, rounded or oval or ribbon-like.

  • Two long aboral tentacles are present.

  • Mouth is narrow and pharynx is small.

Order 1 - Cydippida

  • Body is oval or rounded

  • Two long branched tentacles are present and they can be retracted into sheaths

  • Branches of gastro–vascular system are terminating blindly

  • Examples: Hormiphora, Pleurobrachila, etc.

Order 2 - Lobata

  • Body is laterally compressed

  • Two large oral lobes or lappets and four pointed processes or auricles are present

  • Tentacles are many, non–retractile without sheaths.

  • Stomodaeal and meridional vessels unite with one another

  • Examples: Deiopea, Bolinopsis, etc.

Order 3 - Cestida

  • Body is laterally compressed and ribbon-like

  • Two main tentacles and may lateral tentacles are present

  • Four rows of rudimentary comb plates are present

  • Meridonal and stomodaeal vessels anastomose

  • Examples: Cestum, Velamen, etc.

Order 4 - Platyctenea

  • Body is worm-like and compressed in oral-aboral axis

  • Tentacles with sheaths are present

  • Comb rows or swimming plates are present only in larva

  • Meridional canals are absent, but there is a system of branching peripheral system

Class 2 - Nuda

  • Body is large thimble-shaped or conical

  • Tentacles are absent

  • Mouth is wide and pharynx is large

  • The meridional vessels are produced into a complex system of anastomosing branches

  • Example: Beroe

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