Argon.

Inert gas contained in very small quantity in atmosphere, used to fill electric light bulbs.
Colorless, odorless, nonmetallic, gaseous element, symbol Ar, atomic number 18, atomic weight 39.948. It is grouped with the inert gases, since it was long believed not to react with other substances, but observations now indicate that it can be made to combine with boron fluoride to form compounds. It constitutes almost 1% of the Earth's atmosphere, and was discovered 1894 by British chemists John Rayleigh (1842–1919) and William Ramsay after all oxygen and nitrogen had been removed chemically from a sample of air. It is used in electric discharge tubes and argon lasers.A colorless and odorless inert gas; one of the six inert gases; comprises approximately 1% of the earth's atmosphere; SYN. Ar, atomic number 18.

Or paper nautilus; Octopus living in the open sea, genus Argonauta. The female of the common paper nautilus, A. argo, is 20 cm/8 in across, and secretes a spiraled papery shell for her eggs from the web of the first pair of arms. The male is a shell-less dwarf, 1 cm/0.4 in across.