Porpita belongs to the class Hydrozoa.This group contains the Bluebottles and other floating "nasties" that sting humans when they enter the sea.
Just as a bit of extra info, for those that do not know the Bluebottles:
The Bluebottle or Portuguese Man-of-War (Physalia physalis) [that name should be more familiar] is not a single animal but a colony of four kinds of highly modified individuals (polyps). The polyps are dependent on one another for survival.
The float (pneumatophore) is a single individual and supports the rest of the colony. The tentacles (dactylozooids) are polyps concerned with the detection and capture of food and convey their prey to the digestive polyps (gastrozooids). Reproduction is carried out by the gonozooids, another type of polyp.
The float is a bottle or pear-shaped sac that can exceed 15 cm. It is mainly blue, though its upper margin may show delicate shades of green or pink. It is a living, muscular bag that secretes its own gas, which is similar to air. The float has aerodynamic properties and it seems likely that sailing characteristics may be modified by muscular contraction of the crest. Physalia sails at a slight angle downwind and the course is determined by the curvature of the float and the underwater resistance of the rest of the colony. The float may project either to the left or to the right; the left-handed forms sail to the right of the wind and vice versa. Thus, if the sailing angle of one form leads to its stranding on the shore, the others sailing to the opposite side of the wind may escape.
Link to Google images
: The photos from our trip! Overhere! Feel free to use any of these additional letters to correct the spelling of words found in the above post: a-e-t-n-d-i-o-s-m-l-u-y-h-c