Porpita is found in all seas. It is pelagic, floats on or near the surface. Its size ranges from 2 to 10 cm in diameter. It is regarded as an inverted polyp-its body is discoid and contains a flattened disc-like, chitinous shell. The shell contains many air filled chambers and functions as a float or pneumatophore. Each chamber opens to outside by a couple of pores.
Porpita shows polymorphism. At the centre of the float (on the ventral side) there is a single large gastrozooid (nutritive). It encloses stomach and bears mouth. From the edge of the float hangs many dactylozooids. These contain nematocysts.
These are meant for defence. Many gonozooids (reproductive) hang in between the gastrozooid (or hydranth) and dactylozooids. The gonozooids bear a terminal opening and gonophores. Gonophores are produced as asexual buds. These are medusa-like and contain bell, velum, radial canals and a manubrium. They do not contain mouth, tentacles and sense organs. Gonads develop in the gonopores. The gonophores are set free in the sea water where they discharge gametes and die.
Porpita contains two systems of canals. One is endodermal and the other is ectodermal tracheal system. The tracheal system opens into the air chambers by numerous pores. Air is pumped into the air chambers by the tracheal system.