Pteropods for example, are highly modified gastropods, or snails- some of which have given up their shells in trade for a 'flying' lifestyle, with two beating fins or wings. Salps are the open ocean forms of ascidians ("sea squirts") that use the powerful water pumping system evolved originally for filter feeding, for 'jet propulsion' in the open waters.
The system truly belongs to the jellies- both the Medusae ("jellyfish") and the Ctenophores ("comb jellies"). In the deep water, without obstacles to bump into, the jellies have taken on a diversity of form unrivaled in the blue. Some are 'traditional' and have a rounded body with hanging tentacles. Others are not so conservative, and resemble nothing so much as actively moving lotus blossoms, fanning open 'petals' as they drop and recoil tentacles in an effort to catch even smaller members of this world.