Body Forms and Life Cycle
Cnidarians are polymorphic and alternate between medusa and polyp body forms throught their lives. Polyps tend to be asexual (budding) and are also sessile. They are attached to a substrate at their aboral end, explaining why they are sessile. There cylindrical body has a mouth at one end that is surrounded by tentacles, their mouth and tentacles face upwards. Medusa are dioecious and are motile, since they move with gentle pulsations off the body wall. Tentacles are also found on the medusa. The medusa resembles an inverted bowl, its mouth faces downwards. In a medusa the mesoglea is more abundant.
A medusa may produce a sperm and an egg, through fertilization the sperm and egg becomes a zygote. The zygote then develops into a blastula. Surface cells migrate to the interior of a blastula and the gastrodermis. The blastula continues to develop, and later on it becomes a planula, free swimming larva. The planula swims freely for a short while until it eventually attaches to a substrate. The polyp begins to form once interior cells from the planula split and develop the gastrovascular cavity. As the polyp matures it may reproduce asexually through budding. During budding a medusa bud may form. This bud is able to break free and become a medusa. The cycle may then be repeated.
A medusa may produce a sperm and an egg, through fertilization the sperm and egg becomes a zygote. The zygote then develops into a blastula. Surface cells migrate to the interior of a blastula and the gastrodermis. The blastula continues to develop, and later on it becomes a planula, free swimming larva. The planula swims freely for a short while until it eventually attaches to a substrate. The polyp begins to form once interior cells from the planula split and develop the gastrovascular cavity. As the polyp matures it may reproduce asexually through budding. During budding a medusa bud may form. This bud is able to break free and become a medusa. The cycle may then be repeated.