Cnidaria: Definition, General Characters, Identifying Characters, Examples

Phylum Cnidaria is the group of multicellular animals belonging to the sub-kingdom of Enterozao. According to tissue differentiation, the animals of this group are two types diploblastic and triploblastic.

In the early days, cnidarians and ctenophores would be considered the animals of a single phylum named Coelenterata.

Hatschek in 1888 separated the Ctenophores from the group Coelenterata. Then the Coelenterata to be divided into two separate phyla – Cnidaria and Ctenophora. In Cnidaria, most animals are marine.

The name Cnidaria is derived from the Greek word Cnidos. “Cnidos = thread

Definition of Cnidaria

Multicellular aquatic animals with diploblastic, coelom free, and containing coelenteron, are called Cnidaria.

Figure: Labeled diagram of Obelia sp.

General Characters of Cnidaria

  • Their bodies are multicellular and radially symmetrical.
  • The animals are diploblastic. The body has two layers called ectoderm and endoderm.
  • Like jelly, mesoglea consists of two layers.
  • Inside the body is the gastrovascular cavity, called the coelenteron. They are called the Coelenterata because of the presence of the coelenteron.
  • They have nematocysts in the cnidoblast cells of the body.
  • There is no anus in the body.
  • The oral cavity or hypostome is surrounded by many tentacles.
  • Both intracellular and extracellular digestion system is present.
  • Alternation of generation are asexual polyploid and sexual medusa may be noticed in the life cycle.
  • The planula larva is present in the life cycle.
  • The animals of phylum-cnidaria are marine, though some are freshwater form.
Figure: Labeled diagram of Physalia sp.

Identifying Characters of Cnidaria

  • Tissue layer: Animals in this Phylum have two cell layers(diploblastic) in the embryonic state, e.g., ectoderm and endoderm.
  • Body cavity: Gastrovascular cavity or coelenteron present in the body.
  • Digestion: Digestion may be intracellular or extracellular.
  • Cnidoblast cell: The tentacles contain numerous cnidoblast cells in the animals of the phylum-cnidaria.
  • Habitat: The animals are generally marine in this Phylum.


Some common animals of the phylum-cnidaria are Hydra, Aurelia (jellyfish), Obelia, Metridium (Sea anemone), Pennatula, Corallum, Astraea, Gorgonia, etc.

Figure: Labeled diagram of Hydra sp.

Questions and Answers

1. Nematocysts are a characteristic feature of the phylum

a. Annelida
b. Coelenterata
c. Porifera
d. Echinodermata

Answer: b. Coelenterata

2. In Hydra, the lining of the gastrovascular cavity does not contain

a. Cnidoblasts
b. Sensory cells
c. Interstitial cells
d. None of these

Answer: a. Cnidoblsts

3. Metagenesis is seen in the life cycle of

a. Hydra
b. Sea anemones
c. Aurelia
d. Obelia

Answer: d. Obelia

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