Trilobites were among the most numerous and varied marine animals of their time, ranging in length from about a quarter of an inch to over three feet. Like other arthropods, they could grow only by casting off their old shells. (The best known example of an arthropod outgrowing its shell today is the soft-shell crab.) So, many trilobite fossils are actually the cast off shells and not the animal itself.
The name trilobite refers to the three (tri-) sections of the body, the easiest way to identify a trilobite fossil. Trilobites also are known for their extremely well developed eyes.