There is an outer cellular epidermis , an inner cellular gastrodermis and a complex gelatinous layer between called the mesoglea . The second life stage is a polyp , which is attached to the substrate . It is a hollow and cylinder-shaped, with a mouth surrounded by tentacles at the top. In colonial forms, such as coral animals and zoanthids , the column extends as a tube to connect all the members of the group. If there is both a medusa and a polyp in the life cycle, the medusa develops as part of the colony, but breaks away and swims to grow to sexual maturity.
Most medusa have separate sexes.
The eggs are fertilised after they are shed into the water. The fertilised eggs usually grow into the polyp form, which in turn may break off parts to form multiple small medusas in a process of asexual division. For some Cnidaria forms, the juveniles may remain in the coelenteron and be brooded there, such as in the Waratah Anemone , Actinia tenebrosa . In Cnidaria there are groups that are primarily medusoid in life style, with only a brief, insignificant polyp stage, and others that are primarily polyps, with only a brief medusoid life-stage. The above description is a simplified cnidarian life story and there are many variations within the different groups.