In the poem "The Chambered Nautilus" by Oliver Wendell Holmes, the poet has found a broken nautilus shell on the beach. The nautilus is a type of mollusk. It is also a cephalopod similar to octopuses and squids in that it has numerous tentacles. However, it is unique among cephalopods because it has a shell that it uses for protection. The shell of a nautilus has numerous chambers, and the animal lives in the outermost one. As a nautilus ages and becomes larger, it grows a new chamber on the outside of its shell into which it moves its internal organs, and the inner chambers are filled with gas to help maintain buoyancy. The interior of the shell is lustrous like a pearl, and the animal is sometimes referred to as a pearly nautilus.
Because the nautilus that the poet finds upon the beach is broken open, he can see inside to the pearly chambered interior. The beauty and the intricacy of the shell inspire him to compare it to a ship and to a mansion.
The "ship of pearl" that the poet refers to in the first and second stanzas is the nautilus shell, which he imagines as a sailing ship. He makes it clear that he is only using his imagination when he uses the phrase "poets feign." According to the Merriam-Webster Dictionary, to feign is "to assert as if true; pretend." So, Holmes is pretending that the broken nautilus is a ship of pearl. It sails the main, which means that it sails upon the sea, until it encounters sirens singing. In Greek mythology, sirens were creatures that looked like beautiful women whose singing enchanted sailors and caused them to shipwreck upon reefs or coasts. In the second stanza, the poet writes that the ship of pearl is wrecked, suggesting that it succumbed to the songs of the sirens and ran ashore.
In the third stanza, the poet likens the shell to a home with a "shining archway" and a "door" through which the creature can move to the new chamber it creates and leave the old behind. Finally, Holmes compares the nautilus continually building new chambers to the soul creating ever-larger metaphorical mansions of thought and deed, until finally the shell is abandoned at death.