Reproduction and development in Cnidarians
Cnidarians are usually dioecious, that is, the sexes are separate, but may be hermaphrodite. Gametes are usually shed into the seawater, where fertilisation occurs.
In general, medusae produce gametes, but in instances where only the polyp occurs, (e.g. in anthozoans) gonads are borne on the projecting internal septa or (e.g. in hydras) as protruberances on the outside of the body wall. Often the medusa is reduced in size and never leaves the parent colony in these cases the eggs are fertilised in situ by free-swimming spermatozoa.
After fertilization the zygote divides, giving rise to cells of equal size; cleavage is said to be radial and indeterminate, for it is possible to remove any cell without damage to the embryo. (This should be contrasted with the Ctenophora.)
The cell ball develops into a ciliated larva or planula which is found swimming in the plankton. The mass of cells then forms two layers with a central cavity and the minute planula settles on the substratum where it develops into a polyp.
When there is a medusa stage only in the life history, the planula often develops into an actinula, a tentacled larva, which then develops into a medusa. In some cases, eggs are retained by the parent in special brood-pouches from which the progeny emerge as young adults.