The life cycle of a Ctenophora starts with a sperm and egg (between two different individuals or the same individuals) joining together to create a zygote. The zygote then continues to divide to create a larva, which attaches itself to a substrate, or the surface of a biotic or abiotic factor. It continues to grow using asexual budding. Eventually, the Ctenophora becomes an adult and, with a reliable food source, continue to rapidly produce sperm and egg cells. Development of the fertilized eggs is direct, in other words there is no distinctive larval form, and juveniles of all groups generally resemble miniature cydippid adults. In the genus before the juveniles, like the adults, lack tentacles and tentacle sheaths. In most species the juveniles gradually develop the body forms of their parents. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae, as they live among the plankton and thus occupy a different ecological niche from their parents and attain the adult form by a more radical metamorphosis, after dropping to the sea-floor.