Swimming in the ocean at one stage of life and passing the rest of life vagrant or sessile on the sea-bottom, as part of the diatoms and Oscellaria, the planktonic fucoids, the metagenetic medusæ, some Turbellaria and annelids, the pelagic larvæ of hydroids and corals, and many helminths, echinoderms, Acalepha, and gastropods.