Current Affairs 11th Class

Phylum-Cnidaria (Coelenterata)

Category : 11th Class

(Gk. knide = nettle or stinging cell)

Brief History : Peyssonel (1723) and Trembley (1744) proved these to be animals. Hence, Linnaeus (1758), Cuvier (1796) and Lamarck (1801) included these under ‘Zoophyta’, together with sponges. Leuckart (1847) included sponges and cnidarians under his phylum Coelenterata. Finally, Hatschek (1888) divided “Coelenterata” into three phyla–Spongiaria (= Porifera), Cnidaria and Ctenophora.

General characters

(1) Coelenterates are radially symmetrical animals with tissue grade of body organization.

(2) All the members of this phylum are aquatic, mostly marine.

(3) They are solitary or colonial, sedentary or free swimming.

(4) The body wall is diploblastic. It is made up of two layers of cells, namely the ectoderm and the endoderm with a non-cellular layer called mesogloea in between.

(5) Cnidarians exhibit diamorphism with polypoid and medusoid stage (Metagenesis or alternation of generation).

(6) Asexual phase is generally polyp and sexual phase is medusa.

(7) Coelom is absent; Hence coelenterates are acoelomate animals.

(8) A gastrovascular cavity or coelenteron is present. It can be compared to the gut of higher animals.

(9) Mouth is present but anus is absent (blind-sac body plan). Mouth is surrounded by tentacles.

(10) The most characteristic feature of coelenterates is the presence of nematocysts or stinging cells.

(11) Digestion is extracellular as well as intracellular.

(12) Respiratory, excretory and circulatory system are absent.

(13) Primitive nervous system with synaptic or non-synaptic nerve net but no brain.

(14) Sense organs are statocysts (tentaculocysts), ocelli and olfactory pits.

(15) Reproduction both asexual and sexual.

(16) Development is indirect as there are one or two larval forms, Planula (Obelia) and Ephyra (Aurelia).

Classification of coelenterata : On the basis of the dominance of medusoid or polypoid phase in the life cycle, phylum coelenterata is divided into three classes –

Class 1. Hydrozoa (Gr. hydros, water, zoios, animal)

(1) Hydrozoa are solitary and fresh water or mostly colonial and marine, sessile and free-swimming forms.

(2) They exhibit tetramerous or polymerous radial symmetry.

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

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

(5) Skeleton or horny structure is horny perisarc in some forms, while coenosarc secretes a skeleton of calcium carbonate forming massive stony structure or coral in other forms.

(6) They exhibit polymorphism. There are two main types of zooids, the polyp and medusa. Medusa is provided with true muscular velum.

(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) Both polypoid and medusoid stages present.

Examples : Hydra, Tubularia, Bougainvillea, Hydractinia, Eudendrium, Pennaria, Obelia, Sertularia, Plumularia, Companularia, Millepora, Stylaster, Geryonia, Physalia, Porpita, Velella, Pericolpa, Periphylla, Cynaea, Rhizostoma or Pilema Cassiopeia, etc.,

Obelia is trimorphic and marine colony.
Hydranth of obelia bears twenty four (24) tentacles while medusa bears sixteen (16) tentacles in addition to tentaculocysts.

Class 2. Scyphozoa (Gr. skyphos, cup, zoios, animal)

(1) Scyphozoa include large jellyfishes or true medusae.

(2) They are exclusively marine.

(3) Medusae are large, bell or umbrella-shaped and without true velum. They are free swimming or attached by an aboral stalk.

(4) Marginal sense organs are tentaculocysts.

(5) Polypoid generation is absent or represented by small polyp, the scyphistoma which gives rise to medusae by strobilization or transverse fission.

(6) Gastrovascular system is without stomodaeum, with gastric filaments and it may or may not be divided into four inter-radial pockets by septa.

(7) Mesogloea is usually cellular.

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

Examples : Lucernaria, Haliclysus, Aurelia, Rhizostoma, Charybdea, Periphylla, Chrysaora.

• Rhizostoma is a polystomous scyphozoan with many mouth bearing structures called scapuletes.

Class 3. Anthozoa (Actinozoa)

(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 oral end of the body is expanded radially into an oral disc bearing hollow tentacles surrounding the mouth in the centre.

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

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

(7) Mesenteries bear nematocysts at their free edges.

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

(9) They are exclusively marine, many forms corals.

Subclass 1. Alcyonaria (Octocorallia)

(1) These are colonial marine forms.

(2) Polyps are long or short cylinders terminating orally into a flat circular oral disc having the oval or elongated mouth in the centre.

(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 comprised of calcareous spicules either calcareous or horny in nature.

(7) Polyps are dimorphic in some forms.

Examples : Tubipora, Clavularia, Alcyonium, Xenia, Heliopora, Gorgonia, Corallium,Testudo, etc.,

Corals form rocks in the sea, called the coral reefs. The largest coral reef is the great barrier reef which is 1200 miles long and surrounds Australia complete.