COELENTERATA- LIFE CYCLE OF OBELIA

 

• In obelia colony, reproduction takes place by asexual methods and produces polyp or hydranth and blastostyle.

1. Polyp or hydranth
2. Blastostyle

• Both are asexual zooids. These two zooids represent asexually generation. Blastostyle, produces medusa. Medusa is reproductive zooids. In medusa four gonads are present.
• In medusa, gonads are either male or female. Therefore, medusa is unisexual.

Gonads

In medusa, gonads are found in sub-umbrellar surface. Gonads are per radial in position. Each gonad is round and formed of interstitial cells. Interstitial cells are present between ectoderm and endoderm.

In male medusa, interstitial cells forming, spermatozoa but in female medusa interstitial cells produces ova.

After the maturity of female or male gonad, ectoderm raptures, and spermatozoa or ova releases in water and swims freely in sea water.

Fertilization

The sperm of one medusa swims in water and reaches to the ova of other medusa. Sperms and ova together fuses. In this way cross fertilization takes place.

Generally, sperms reach to ovum by water current. Sperm is male gamete and ovum is female gamete. Male gamete and female gamete unite to form single zygote.

Development

Zygote divides holoblastic manner. Division of zygote is called cleavage.

Cleavage -  

Zygote first divide to form four celled stage.

First and second cleavage is vertical.

Third cleavage is horizontal, forming eight celled stage.

Then, divides and redivides to form hollow structure called Blastula stage.

 The cavity of Blastula is called Blastocoel.

 The wall of Blastula is one cell thick.

Stereo gastrula 

The cells of Blastula divide and redivide to fill up the Blastocoel cavity. It is called solid gastrula. It is also called stereo gastrula. The outer wall of gastrula is called ectoderm and the inner wall of gastrula is called endoderm. After sometime it elongates and ectodermal cells develops cilia. Gradually in the center a cavity is formed. It is called coelenteron. Now, gastrula changes into planula larva. Planula larva swims in water freely.

Hydrula 

After some times the free swimming planula larva fixes with the free swimming planula larva fixes with any substratum (stone or wooden logs). Now changes took place in planula. The anterior part of planula forms basal disc, but the posterior part of planula forms manubrium. The terminal part of manubrium breaks to form mouth. Many tentacles develop around mouth. This stage is called hydrula. The basal part of hydrula, is called Hydrorhiza. Hydrorhiza grows on substratum and gives vertical branches.  vertical branches grow upward. From the vertical branches buds arises. Due to budding polyp and blastostyle are formed. Gradually, a complex obelia colony is formed. Blastostyle, slowly and slowly form medusa buds and starts sexual generation. Medusa is swimming stage in obelia. It works for dispersal. It is essential for survival.

Alternation of generation or metagenesis

Generally, in the life cycle of particular organism, after sexual generation, asexual generation is found. The beginning of sexual generation starts from sex cells. Sex cells are formed in parent body but asexual generation are formed after division of sexual generation.

The individual of asexual generation don’t bear gonads.

Sexual generation are formed by division of asexual generation. In sexual generation, gametes (female & male) are formed.

 This process in which sexual generation and asexual generation is found alternately is called Alternation of Generation. Today, it is called metagenesis.

In the life cycle of obelia, it is a good example of alternation of generation. This process is going on continuously.

The hydra like colony of obelia is showing asexual generation. But medusa forms sexual generation. The hydroid colony of obelia bears no gametes or sex cells. The colony reproduces, asexually by budding, forming polyp and blastostyle. 

Blastostyle produces medusa. Medusa by sexual reproduction produces gametes. Formation of gametes takes place by gametogenesis.

After the fusion of male and female gametes, zygotes are formed. In this way regularly metagenesis occurs. Although, in obelia true alternation of generation is not found.

Because, hereafter diploid generation, haploid generation is not found. In obelia only gametes are haploid and sexual medusa bears gonads. Gonads are diploid like hydroid form.

Conclusion

In this way medusa is sexual form. It bears sex organs and swims freely. It produces male or female gametes. Sexual and asexual generation is clearly seen in obelia.