Describe the different cnidaria life cycles.

Cnidarians exhibit Alternation of generation which means polyps alternate with medusae and vice versa.

Explanation:

Cnidarians have two basic body forms, one is called the polyp and the other is called the medusa form. Polyp is usually sessile and the body is hydra like i.e. cylindrical with mouth and tentacles facing upwards. The medusa form is free floating or swimming structure like we see in jelly fish. It looks more like a bell or umbrella with mouth and tentacles facing downwards.

In most of the cnidarians it is observed that polyp gives rise to medusa by vegetative budding whereas medusa gives rise to polyp by sexual reproduction. For example in case of Obelia we observe alternation of generations between these two forms. This is also referred as metagenesis.

During sexual reproduction the medusa forms release their gametes into the water which after fertilization forms zygote. The zygote then develops to form a ciliated larva called planula larva. The planula larva swims, settles and grows into a sessile polyp form. Both the sexual and asexual forms are diploid and the only haploid cells are gametes.

In some other cnidarians such as hydra, there is no medusa stage (thus no metagenesis) and there is no larva.