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