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(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.
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.
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
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.
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.
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.
(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.
The body is simple, rounded or oval or ribbon-like.
Two long aboral tentacles are present.
Mouth is narrow and pharynx is small.
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.
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.
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.
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
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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