The organism alive today that is most closely related to the trilobite is the horseshoe crab.

While they don't look much alike on the surface, horseshoe crabs share a common ancestor with trilobites, making them their closest living relative. Both belong to the Chelicerata subphylum, which also includes spiders, scorpions, and ticks.

Here's why they're considered related:

* Similar body plan: Both have a hard exoskeleton, segmented bodies, and a pair of compound eyes.

* Shared evolutionary history: Fossil evidence indicates that trilobites and horseshoe crabs share a common ancestor that lived about 500 million years ago.

Although they share a common ancestor, the two groups have evolved significantly over time, leading to their distinct appearances and lifestyles. Trilobites are extinct, while horseshoe crabs are remarkably unchanged in over 200 million years.