The title of this poem by Oliver Wendell Holmes gives us a clue as to what the speaker is looking at. His imagery is fanciful and elaborate, but it is clear that he is looking at a "nautilus," or shell, even though he imagines it as a "ship of pearl." He conceptualizes the shell as representing the sort of ship poets write about—the sort of ship which might have sailed the open seas alongside such mythical creatures as sirens and mermaids. However, he imagines that the shell is now a "wrecked" ship. It has many chambers within it, which once harbored life. This is true both in terms of the speaker's fantasy but also in reality: the "tenant" of the shell would have been some kind of sea creature, working for years to grow its shell and then eventually leaving it.
The speaker personifies the creature whose "home" was once this shell, suggesting that he worked very hard to build up his abode but then eventually left it, looking for a new home. In the final stanza, a comparison is drawn between this building of a home by a sea creature and the way in which the soul builds "stately mansions." This metaphor represents the spiritual fortification of a person's soul, a building project which will ultimately result in the person being spiritually "free."