Biology,  Zoology

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
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
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
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