Important body parts of medusa are given below:
Medusa is a sexual zooid and is shaped like an umbrella, bell or saucer, measuring 1or 2 mm in diameter.
Outer convex surface of the umbrella is known as ex-umbrella while inner concave surface as sub umbrella. The following are the body parts of medusa:
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(A) Manubrium:
It is a handle-shaped process, hangs down from the centre of sub-umbreller surface of the medusa. It bears four-sided mouth, surrounded by four oral lobes, at its free distal end.
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(B) Velum:
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Velum is a very narrow rudimentary fold that makes circular margin of the medusa. In obelia it is not prominent but well developed in other Hydrozoans.
(C) Tentacles:
The circular margin of medusa bears, highly contractile tentacles. Their swollen bases are called tentacular bulbs, that may lodge the sense organs statocysts and serve as the sites for nematocyst-formation. A young medusa bears 16 tentacles.
(D) Gastrovascular cavity:
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In medusa gastrovascullar cavity starts from mouth and leads into gullet that runs along manubrium.
Gullet followed by a dilated stomach lying at the base of manubrium and central part of umbrella.
Along with circular margin (velum) a circular canal or ring canal runs and is linked with stomach through 4 radial canals. Radial canals run equidistant and at right angles to each other.
(E) Gonads:
Medusa is dioecious, i.e., testes and ovaries are borne by separate individuals. There are 4 gonads, formed as ventral diverticula of each radial canal in the middle.
Each gonad is a knob-like body, has same epidermis and gastrodermis continuous with that of radial canal.
The space between these two layers is filled with a mass of sex cells, which become differentiated into sperms or ova, as the case may be.