Types of individuals in Cnidaria
There are two forms of individuals in Cnidaria:
Polyp is tubular, the oral end being free carrying a whorl of tentacles, and the aboral end attached to the substratum by the basal disc. The polyp is sedentary, benthic type. The medusa is the free-swimming umbrella-like pelagic form, with mouth at the end of the manubrium on the subumbrella side. Though polyp and medusa may appear to be quite different in form, they have basically the same plan with an outer layer of epidermis and a gastrodermis lining the gastrovascular activity with a gelatinous layer of mesoglea in between them. The mesoglea in the medusa is, however thicker (hence the term jelly fishes), and one form can be theoretically derived from the other. The gastrovascular cavity servers as the alimentary canal opening to the outside through just only one opening the mouth.