In this article we will discuss about the classification of phylum cnidaria.

Classification of the Phylum Cnidaria:

1. The Cnidaria are radially symmetrical in oral-aboral axis (biradial in some Anthozoa); no head and segmentation; sim­plest of all metazoans with a low grade of tissue formation.

2. The body wall is composed of two principal layers, an outer epidermis (ecto­derm) and an inner gasrtrodermis (endodem), separated by a gelatinous non-cellular layer, the mesoglea; there are cnidocytes and a few other structures in either or both layers.

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3. The body wall encloses a single inter­nal cavity lined by gastro dermis called coelenteron or gastro vascular cavity, which opens outside only by a single opening, the mouth.

4. Skeleton external or internal, calcic, horny or none; muscle fibres as a continuous or separate mass in epithelia.

5. A diffuse network of un-polarised nerve cells in body wall; but no central nervous system.

6. Reproduction—both asexual and sexual; asexual by budding, leading to exten­sive colony formation and sexual by produc­tion of ova and sperms.

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7. The gonads are only aggregations of developing gametes, no gono-duct is present.

8. Fertilization usually external, and de­velopment leads to free-living, ciliated larva, the planula.

9. All  are aquatic, mostly marine; indi­viduals either solitary or colonial and seden­tary or free-swimming.

The phylum Cnidarian has been di­vided into three classes:

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Class 1. Hydrozoa:

1. The individuals are colonial in most cases, some are polymorphic; a few solitary; marine and fresh water (Hydra); sessile or free; 2,700 species known.

2. The medusae develop from the sessile polyp colony by budding and are always free-swimming unisexual, with a true mus­cular and non-muscular velum and nerve ring; reproduce sexually, forming hydroid colonies through fertilized eggs.

3. The gastro vascular cavity is simple, without stomodaeum or nematocyst bearing gastric filaments and not divided by vertical partitions and mesenteries.

4. Tentacles are usually solid.

5. The ectoderm secretes a cuticular horny layer called perisarc, which provides rigidity and support; perisarc is calcareous and enor­mously thickened forming a massive stony structure or coral.

6. Gonads commonly of ectodermal ori­gin and discharged externally.

7. An alternation of generations or metagenesis is present in most cases; the dominant phase is a polypoid or hydroid stage, usually sessile, branched, colonial and often polymorphic.

8. Embryo is a ciliated planula. Examples : Hydra, Obelia, Bougainvillea, Stylaster, Physalia, Sertularia, Valella, Porpita, etc.

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Class 2. Scyphozoa (True Me­dusae):

1. The individuals are medusoid, polyp stage absent or inconspicuous; exclusively marine forms, commonly known as jelly-fish; about 200 known species.

2. Medusae phase dominant; medusae large, bell or umbrella-shaped and free-swim­ming or attached by an aboral stalk.

3. Gastric tentacles bearing cnidocytes surround the mouth are endodermal in ori­gin and hollow.

4. Velum and nerve ring absent and, in many cases, sense organs are represented by tentaculocysts.

5. A skeleton is entirely absent.

6. Vertical partitions are present in the enteron of some forms and in the larval enteron of others.

7. Gonads are endodermal in origin and discharge their products into the digestive cavity.

8. Polyp when present is nonsexual, known as scyphistoma, gives rise to medusae by transverse fission (strobilation).

9. Development is direct or with an alter­nation of generations.

Examples: Lucemaria, Pericolpa, Aurelia, Pilema, Tamoya, Chironex, Khizostona, Cyanea, etc.

Class 3. Actinozoa or Anthozoa:

1. Solitary or colonial polyps without medusoid stage; exclusively marine; attached or fixed to the substratum; cylindrical body with biradial symmetry; about 6,100 species are known.

2. Oral end is expanded to form an oral disc, bearing six to several hundred hollow tentacles surrounding the mouth; ectoderm and endoderm are separated by a strong mesoglea containing fibres and cells.

3. Vertical partitions are present in the enteron.

4. A skeleton may or may not be present.

5. The free margins of the mesenteries bear coiled mesenteric filaments, which serve the function of gastric filaments.

6. The mesenteries are formed of a double layer of endoderm with a supporting plate of mesogloea.

7. The nervous system is a typical nerve net, central nervous system absent.

8. The sex cells are endodermal in origin and the sexual products are discharged into the coelenteron; asexual reproduction by bud­ding.

9. The planula metamorphoses to adult form.

Subclass i. Alcyonaria or Octocorallia:

a. Entirely colonial polyps in warm coastal waters, with eight pinnately, branched tentacles and eight complete septa.

b. One ventral siphonoglyph and an endoskeleton present.

c. Often brightly coloured and phospho­rescent.

Examples: Clavularia, Telesto, Alocycmium, Tubipora, Heliopora, Gorgonia, Corallum, Renilla, Pennatula, etc.

Subclass ii. Zoantharia or Hexacorallia:

a. Colonial or solitary polyps with usu­ally more than eight un-branched tentacles.

b. Skeleton present in some forms. Examples: Urticina, Adamsia, Metridium, Zoanthus, Cerianthus, Fungia, Madrepora, Antipathes, etc..