Important types of reproduction system in obelia are as follows:
It takes place alternately i.e., diploid asexual stage and haploid sexual stage come one after another regularly. This phenomenon is called alternation of generation.
A. Asexual reproduction in Blastozooid: (Development of Medusa)
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In older blastozooids development of medusae occurs by a process of asexual budding in spring and summer.
First of all, the wall of blastostyle outpushes itself to give rise to small hollow protuberance. It soon grows into a vesicle, connected by a narrow stalk with blastostyle. Apical epidermis of vesicle now splits into two layers.
The inner layer again splits and acquires a small cavity, called bell-rudiment. The vesicle grows in size and simultaneously innermost wall first invaginates and then evaginates to form the prospective manubrium.
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As the bell rudiment enlarges to become sub-umbrellar cavity, manubrium also grows in size. Manubrium further elongates, comes out by breaking double layered outer epidermis.
The broken epidermis makes prospective velum that projects inwards from the margin of umbrella. Velum is permanent and prominent in other genus but vestigeal in obelia.
Later the mouth forms at the apex of manubrium tentacles arise from circumference of umbrella.
Finally stalk ruptures and medusa get detached from the blastostyle. Then it escapes through gonopore of gonangium to lead a free swimming life. In a few months it grows to full size and gets ready for sexual reproduction.
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B. Sexual Reproduction in Medusa :
Medusae, the sexual reproductive zooids or gonozooids are diocious. There are four gonads situated in the middle of each radial canal and formed as ventral diverticula of radial canal. They mature after the medusae escape from gonangia, Each gonad possesses a mass of sex cells in the space between epidermis and gastrodermis.
These sex cells are originally borne in the epidermis of manubrium while medusa remains in the blastostyle.
They migrate into the gonads passing through gastrodermis of the manubrium. Here they undergo reduction division to give rise gamete (ova and sperms). Outer wall (epidermis) of mature gonads ruptures to release the gametes (sperms) in water.
Fertilization:
Ovum remains inside the ovary and sperms may be carried to the female medusa by water currents. Fertilization takes place in situ, and then female medusa dies soon after liberating the zygote.