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 –

         (1) Coelenterates are multicellular organisms

         (2) They have tissue-grade of organization

         (3) The body is radially symmetrical. Radial symmetry is the symmetry of a wheel

         (4) All the members of this phylum are aquatic

         (5) They are solitary or colonial

         (6) polyps and medusa occur in the life cycle.

         (7) The body wall is diploblastic.

         (8) Nematocysts or stinging cells are present.

         (9) Coelom is absent; hence coelenterates are acoelomate animals

         (10) A gastrovascular cavity or coelenteron is present.

         (11) Mouth is present; but anus is absent

         (12) Digestion is extracellular as well as intracellular

         (13) Respiratory, excretory and circulatory system are absent

         (14) Nervous system is diffuse-type, formed or nerve-nets.

         (15) Reproduction is by asexual and sexual methods

         (16) Development is indirect as there are one or two larval forms

         (17) Life history has alternation of generations or metagenesis.

(iv) Classification of coelenterata: Phylum coelenterata is divided into three classes.

(a) Class 1 - Hydrozoa

         (1) Hydrozoa is solitary and fresh water or mostly colonial and marine.

         (2) They exhibit tetramerous or polymerous radial symmetry

         (3) Body wall consists of an outer ectoderm and an inner endoderm separated by a mesogloea.

         (4) Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament

         (5) Skeleton or horny structure is horny perisarc in some forms.

         (6) They exhibit polymorphism.

         (7) Many hydrozoa exhibit alternation of generation

         (8) Reproductive products of sex cells are usually ectodermal in origin and discharged externally.

         (9) Cleavage is holoblastic, embryo ciliated planula.

         (10) Examples : Hydra, Tubularia, Bougainvillea, Hydractinia, Eudendrium, Pennaria, Obelia, Sertularia, Plumularia

         (11) Physalia is commonly known as Portuguese man of war. Aurelia is commonly known as Jellyfish.

(b) Class 2 - Scyphozoa

         (1) Scyphhozoa includes large jellyfishes or true medusae.

         (2) They are exclusively marine.

         (3) Medusae are large, bell or umbrella-shaped and without true velum.

         (4) Marginal sense organs are tentaculocysts

         (5) Polypoid generation is absent or represented by small polyp.

         (6) Gastrovascular system is without stomodaeum, with gastric filaments.

         (7) Mesogloea is usually cellular

         (8) Gonads are endodermal and the sex cells are discharged into the stomach.

         (9) Examples: Lucernaria, Haliclytus

(c) Class 3 - Anthozoa

(1) These are solitary or colonial exclusively marine forms

(2) They are exclusively polypoid. Medusoid stage is altogether absent

(3) Body is cylindrical with hexamerous, octomerous or polymerous biradial or radiobilateral symmetry

(4) The stomodaeum is often provided with one or more ciliated grooves, the siphonoglyphs.

(5) Gastrovascular cavity is divided into compartments by complete or incomplete septa or mesenteries.

(6) Mesenteries bear nematocysts at their free edges

(7) Mesogloea contains fibrous connective tissue and amoeboid cells.

Subclass 1 - Alcyonaria (Octocorallia)

         (1) These are colonial marine forms

         (2) Polyps are long or short cylinders

         (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 mesogloeal cells

         (7) Polyps are dimorphic in some forms.

         (8) Examples: Tubipora, Calvularia, Alcyonium, Xenia, Heliopora, Gorgonia, Corallium, etc.,

         (9) Tubipora is commonly known as organ pipe coral.

Subclass 2 - Zoantharia (Hexacorallia)

         (1) These are solitary or colonial marine forms

         (2) Tentacles simple, rarely branched, hollow cone shaped

         (3) Mesenteries are numerous arranged in the multiple of five or six

         (4) Two siphonoglyphs are commonly present

         (5) Endoskeleton when present is calcareous, derived from ectoderm

         (6) Polyps are usually monomorphic.

         (7) Examples: Actinia, Metridium, Adamsia, Edwardsia, Astraea, Fungia, Zoanthus, Antipathes

         (8) 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

         (1) All the ctenophores are marine.

         (2) They are solitary and pelagic.

         (3) They are transparent.

         (4) They have tissue-grade of organization.

         (5) They have biradial symmetry.

         (6) They are acoelomate animals.

         (7) They are non-segmented.

         (8) Their body-wall is diploblastic.

         (9) The mesogloea contains cells.

         (10) Nematocysts are absent.

         (11) Special adhesive cells called colloblasts are present in all ctenophores.

         (12) The gastrovascular system is well developed.

         (13) Two anal openings are present.

         (14) Skeletal system is absent.

         (15) Excretion and respiration are carried out by diffusion.

         (16) The nervous system is in the form of nerve net.

         (17) An aboral sense organ in present in the form of statocyst.

         (18) Cilia are used for locomation.

         (19) They are hermaphrodites.

         (20) Development is indirect. It includes a cydippid larva.

(iii) Classification of Ctenophora

(a) Class 1 - Tentaculata

         (1) The body is simple, rounded or oval or ribbon-like.

         (2) Two long aboral tentacles are present.

         (3) Mouth is narrow and pharynx is small.

Order 1 - Cydippida

         (1) Body is oval or rounded

         (2) Two long branched tentacles are present and they can be retracted into sheaths

         (3) Branches of gastro–vascular system are terminating blindly

         (4) Examples: Hormiphora, Pleurobrachila, etc.,

Order 2 - Lobata

         (1) Body is laterally compressed

         (2) Two large oral lobes or lappets and four pointed processes or auricles are present

         (3) Tentacles are many, non–retractile without sheaths.

         (4) Stomodaeal and meridional vessels unite with one another

         (5) Examples: Deiopea, Bolinopsis, etc.,

Order 3 - Cestida

         (1) Body is laterally compressed and ribbon-like

         (2) Two main tentacles and may lateral tentacles are present

         (3) Four rows of rudimentary comb plates are present

         (4) Meridonal and stomodaeal vessels anastomose

         (5) Examples: Cestum, Velamen, etc.,

Order 4 - Platyctenea

         (1) Body is worm-like and compressed in oral-aboral axis

         (2) Tentacles with sheaths are present

         (3) Comb rows or swimming plates are present only in larva

         (4) Meridional canals are absent, but there is a system of branching peripheral system

(b) Class 2 - Nuda

         (1) Body is large thimble-shaped or conical

         (2) Tentacles are absent

         (3) Mouth is wide and pharynx is large

         (4) The meridional vessels are produced into a complex system of anastomosing branches

         (5) Example: Beroe