Oldest it can be!

Unusual In Its Own Pod

The ivory nautilus is unusual among living cephalopods in having a large external shell divided into chambers that are punctured by a siphuncle. This odd cephalopod lives within one of its most recent chambers. The nautilus can withdraw into the shell and seal the entrance with a large tough hood. Buoyancy, which is provided by the shell, is controlled by pumping fluid in and out of the chambers using an osmotic mechanism. This pump operates against pressure and a depth limits of the ocean. It can only exceed certain for short periods of time.
The Chambered Nautilus