Nautiloida
 
The nautilus is one of the most beautiful and astounding of animals. Being the only Cephalopod to have maintained an external shell (a general feature of molluscs) they are thought to have evolved in the late cambrian and are the only survivor of a larger group of similarly pelagic and externally shelled cephalopods.
 
Their almost surreal motionless stability in the water column is thanks to the unique buoyancy system of their chambered shell: as the animal grows, its shell grows, and adds more chambers inside. Adult Nautilus will generally reach about 30 individual chambers. All of these are connected by a “siphuncle” a kind of tubular structure which can, through osmosis either add fluid to the chambers or extract fluid, thus balancing the liquid to gas ratio to very fine degrees to achieve neutral buoyancy at different depths.  This comes in handy, as they are typically found on coral reef slopes, where nautilus have been observed up to a depth of 703 meters.
 
They have long life spans with 20 year old nautilus being recorded (most other cephalopods are live fast die young kind of animals). Having survived 5 mass extinctions, they are now threatened by disturbances to their habitat, ocean acidification (affecting their ability to form calcified shells) and the collection of them for their shells as tourist trinkets or home décor.

For T-shirts and art prints of the Nautilus in my Etsy store click on the images below:

For museum quality art prints (starting from 18.55 USD):

For frontal print sweatshirts (starting from 34.19 USD):

For frontal-print t-shirts (starting from 20.97 USD):