Biology Sikshya

     Gr., koilos=cavity; enteron=intestine

• Most of them are marine, but a few fresh water form also exist.

• They may be solitary or colonial and sessile or free-living.

• Diploblastic animals, having outer epidermis, inner gastrodermis and between these two layers mesoglia is found.

• Body is radially symmetrical.

• Body wall encloses a central cavity known as gastrovascular cavity or coelenteron. Food is digested in this cavity.

• Cell-tissue grade of body organization is found.

• Coelom is absent.

• Locomotion takes place by smooth muscles fibers of the epithellium.

• Head is absent.

• Body wall contains undifferentiated interstitial cells and stinging cells i.e., nematocyst.

• Nematocyst functions as offensive, defensive, food capturing and adhesive organ.

• Mouth (Hypostome) is present at the raised portion and is surrounded by numerous tentacles, which acts as defensive and food capturing organ.

• Polymorphism is widely marked.

• Alternation of generation is usually present.

• Respiration takes place by normal body surface.

• Digestive system is incomplete type. Both .extracellular and intracellular digestion occur.

• Excretory system is absent.

• Circulatory system is absent.​

• Nervous system is in the form of a network of unpolarised nerve cells on the body wall.

• Sense organ may include statocysts, light sensitive organ called ocelli or may be a functional eye spot.

• Asexual reproduction occurs by budding and sexual reproduction occurs by the formation of sperm and ovum.

• Development is indirect through larval form i.e., Planula or Stereogastrula.

• Fertilization may be external or internal.

• They may be monoecious or dioecious.

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