Phylum Cnidaria/Coelenterata

Kingdom: Animalia
Habitat: aquatic, mostly marine.
Habit: solitary or colonial. Each individual is known as zooid.
Symmetry: radially symmetrical
Grade of organization: tissue grade of organization.
Germ layer: diploblastic, outer ectoderm and inner endoderm. Mesogloea separates these two layers.
The body has a single opening called hypostome surrounded by sensory tentacles.
Coelom: gastrovascular cavity or coelenteron.
Nematocyst: organ for capturing and paralyzing pray, present in tentacles
Nutrition: holozoic
Digestion is both intracellular and extracellular.
Respiration and excretion are accomplished by simple diffusion.
Circulatory system: absent
Nervous system: poorly develop
Many forms exhibit polymorphism ie. Polyp and medusa
Polyps are sessile, asexual stage
Medusa are free swimming, sexual stage
Metagenesis: asexual polypoid generation alternate with sexual medusoid generation
Asexual: by budding
Sexual: by gamatic fusion
Fertilization: internal or external
Development: indirect with larval stage
(Koilos: hollow, enteron: cavity)

Classification of Phylum Cnidaria/Coelenterata

The phylum coelenterate is divided into three classes on the basis of development of zooids:

Class1: Hydrozoa

(Hydra; water; zoon: animal)

  • Habitat: mostly marine, few are fresh water
  • Habit: some are solitary and some are colonial
  • Asexual Polyps is dominant form
  • Medusa possess true velum
  • Mesogloea is simple and acellular
  • Examples: Hydra, Obelia, Physalia physalis (Portuguese man of war), Tubularia
Hydra (Hydridae)
Hydra (Hydridae)
Physalia physalis
Physalia physalis



Class 2: Scyphozoa or Scyphomedusae

  • Habitat: exclusively marineHabit: solitary, freely swimming
  • Medusa is dominant and it is Large bell or umbrella shaped
  • Polyps is short lived or absent
  • Mesogloea is usually cellular
  • Examples: Aurelia aurita (Jelly fish), Rhizostoma
Aurelia aurita
Aurelia aurita (Jelly fish)

Rhizostoma pulmo (Barrel_jellyfish)

Class 3: Anthozoa or Actinozoa

(Anthos: flower; zoios: animal “flower like animals”)

  • Habitat: exclusively marine
  • Habit: Solitary or colonial
  • Medusa stage is absent
  • Mesogloea contains fibrous connective tissue and amoeboid cells.
  • Examples: Metridium (sea anemone), Telesto, Tubipora, Xenia
Metridium (sea anemone)

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