Jellyfish zoological name Aurelia. Structurally Aurelia
is a unisexual organism. Sexual dimorphism is absent. In the male 4 testis are present and in the female medusa 4 ovaries are present. Testis will produce sperms. Ovaries will produce ova
. The ova are. Released into gastric pouches.
Through in current water sperms will enter into the gastric pouches of the female.Sperms will fertilise the ova in the gastric pouches. Fertilization is internal. The zygotes throughout-going water will enter into the grooves of the oral
arms There it undergoes development. The zygote undergoes holobiastic and equal cleavage. It becomes morula, and it transforms into a hollow blastula. Blastula shows a fluid filled cavity called segmentation cavity (or) blastocoel. By the invagination of the blastula layer into the blastocoel and two layered gastrula is formed. The embryo elongates and its outer layer becomes ciliated. It becomes planula larva
This planula larva will enter into water and shows free swimming.
The planula larva attaches itself to a stone or a plant Then it transforms into a hydra like polyp
. At the free distal end mouth is present. Around the mouth 16 tentacles are present. This stage
is called Scyphistoma
. The endoderm of the enteric cavity will develop 4 inter radial longitudinal ridges. These are called gastric ridges.
This Scyphistoma will undergo transverse fissions called strobilation. These segments are called ephyra larva. These ephyra larvae will get detached and liberated into the water.
- It is a young medusa form of aurelia.
- It has tetramerous symmetry.
- The edge of the larva will show S bifid lobes (4 are perradial and 4 are interradial in position). In the 8 adradial positions clefts are present.
- On the exumbrellar side the stomach becomes closed, while on the subumbrellar side it opens Its edges grow out to form a short manubrium bearing four sided mouth at its apex.
- The gastric ridges are replaced by gastric filaments.
This ephyra larva swims in the water and feeds on minute organisms. It slowly transforms into adult. The adradiai regions will grow rapidly and ephyra becomes saucer shaped adult by developing 8 tentaculocysts, 4 oral arms, and many marginal tentacles.
Alternation of generations:
The free swimming adult aurelia medusa is asexual zooid. Male and female medusae are separate. Male medusa will produce sperms and female medusa will produce ova. Fertilization is internal and zygote is formed. It becomes planula larva. It freely swims in the water and attaches to a substratum. It becomes seyphistoma. It represents asexual stage. It reproduces asexually by strobilation and produces ephyra larvae. They transform into medusa.
In the life cycle sexually reproducing medusa will alternate with asexually reproducing scyphistoma. Thus it shows alternation of generations. But the concept of alternation of generations in Aurelia is not true because the two alternating stages are diploid, hence it is called metagenesis.