Some cnidarians have a life cyce wih an alternation of generations between asexual reproduction and sexual reproduction; while asexual reproduction provides a large increasing of (identical) individuals, sexual reproduction mantain a genetic pool variety.
The generation supposed to reproduce asexually is polypoid; the generation supposed to reproduce sexually is medusoid.
Tipically, medusae produce male and female gametes, which fuse together making zygotes; a zygote becames an egg from which hatches a free swimming larva called planula; the planula reaches the sea floor and developes into a polyp; the polyp then becames a strobila (segmented polyp), wich reproduces asexually by strobilation, segmenting itself and producing many ephyras, which can be released one at a time or all together at the same time; each ephyra maturates into an adult medusa, reaching the reproductive maturity.