Dave Bonta's poem looks closely at a purple sea urchin and keeps finding that its spines do more than we expect. They fend off predators, but they also catch drifting algae, help the urchin dig into rock, and even seem to sharpen its vision. The tone starts out factual, almost like a field guide, with a hint of quiet delight in each new detail.
Then the poem shifts. The urchin has no brain and no eyes, yet the speaker reasons that it is "all brain" and "all eye." This turns a biological oddity into something strange and moving. The final image is the most striking: an animal turning in the hollow it carved for itself, living for a century, "unable ever to shut." The mood becomes awed and a little uneasy. The speaker seems to admire the urchin's constant awareness, while also feeling how exposed and vulnerable that way of living is.