True jellyfish (Cnidaria, Scyphozoa) are radially symmetrical, usually tetraradial or octoradial, i.e. segments repeating 4 times or 8 times around the body.  They are planktonic, although most swim actively in the water column.  They have stinging cells (nematocysts) which in several species can penetrate human skin and so impart a sting.  Six species occur in Highland waters and stranded on beaches.

Their relatives the colonial Hydrozoa, most famously the Portuguese Man o' War Physalia physalis, are often mistaken for jellyfish.  Our most familiar marine hydrozoan is the By-the-wind Sailor.

Maps show records on the NBN Atlas held by HBRG, Outer Hebrides Biological Recording, and Lorn Natural History Group.