Cnidaria reproduce sexually or asexually. Some cnidaria are monoecious and only have one sex, while others are dioecious and contain both sexes. Cnidaria is a phylum of animals that includes jellyfish, coral and sea anemones.

The grooved brain coral is dioecious, though it reproduces both sexually and asexually. The eggs of the coral are fertilized internally and then broadcast into the water. There, they hatch into larvae that begin to reproduce asexually through budding.

The sea anemone reproduces either by another sea anemone budding out of the parent body or by releasing eggs and sperm. A fertilized egg hatches into a free-swimming larva called a planula, which eventually settles down on a hard surface and grows into another sea anemone.

The moon jellyfish's gonads can be seen in its nearly transparent bell. Moon jellyfish are either male or female. Males release sperm into the water, and the females bring them into their bodies to fertilize eggs. The fertilized eggs hatch into planulae. Soon after hatching, the planula attaches to a surface where it transforms into a polyp. A polyp can live for some years. After a time, the polyp develops ephyrae, which are tiny jellyfish clones. The ephyrae break off and grow into adult moon jellyfish.