LIFE HISTORY OF OBELIA

 - In Obelia medusae bear gonads, serve to bring about sexual reproduction and dispersal of species. Both sperm and egg are shed into the sea water by breaking of the epidermis of the gonads which results in the death of medusae.

- The life history of Obelia show following stages : By chance both sperm and ova comes in contact in the sea water and results in external fertilization. After fertilization, zygote undergoes holoblastic cleavage and forms blastomeres. It converts zygote into hollow spherical blastula enclosing cavity, blastocoel.

- Some of the blastomeres cut off from inner surface and immigrate to opposite pole in blastocoel. This process is called delamination. Immigrated cells ultimately fill up the blastocoel. Outer layer of the cells is termed as ectoderm and inner mass of cells form endoderm. This embryonic stage is called stereogastrula i.e., solid gastrula.

- Gastrula elongates and its ectodermal cells acquire flagella. This stage is called planula. It has broad anterior end and narrow posterior end. It swims in water from few hours to several days. - During free swimming endoderm develops a cavity enteron and both the layers show cellular differentiation. Planula now settles on some object and its flagella disappears.

- Planula attached end flattens into a disc and free end develops tentacles with manubrium enclosing mouth. This stage resembles Hydra so is termed as hydrula.

- Hydrula now starts feeding and initiates a new colony formation, made up of polyps and medusae.