a class of extinct marine arthropods. The Trilobita appeared early in the Cambrian and became extinct in the Middle Permian. The elliptical body measured 3–10 cm in length, sometimes as little as 5 mm or as much as 45–70 cm. It consisted of cephalic, truncal, and caudal segments and was divided by two longitudinal furrows into axial and lateral parts.

The body was covered on the dorsal side by a chitinous integument strengthened by mineral secretions, and on the ventral side by a thin noncalcified membrane. The cephalic segment resembled a shield whose axial portion (glabella) was smooth or with one to four furrows that divided it into pleurae. On the sides of the glabella were cheeks with eyes that were faceted (with as many as 15,000 densely packed polygonal lenses) or aggregate (with as few as two or as many as 300–400 biconvex lenses). Some trilobites did not have eyes.

On the underside of the cephalic shield, trilobites had one pair of multisegmented antennae, five pairs of double-branched extremities, and a mouth. The stomach, which had hepatic processes, was located under the glabella; the small intestine extended from it to the caudal segment. The trunk consisted of two to 44 mobile, articulated segments; the caudal portion consisted of few segments. Only the Early Cambrian olenellids had a single caudal segment. The integuments of the caudal segment frequently coalesced to form a caudal scutum of variable size and shape. Double-branched extremities attached to the truncal and caudal segments enabled the arthropods to creep on the bottom and swim.

The trilobites were dioecious. The larvae probably maintained a planktonic mode of life and later adapted to life on the bottom. Some trilobites coiled up in the presence of danger. There were two subclasses: Miomera and Polymera. Miomera, consisting of approximately 80 genera, had two or three truncal segments, while polymers, embracing more than 1,400 genera, had between five and 44 segments.

The trilobites flourished in the Cambrian. At the end of the Ordovician the total number of genera decreased sharply, and the monomers became extinct. The number of genera continued to dwindle in the Silurian and Devonian, and by the Middle Permian all were extinct. The trilobites are very important for studying the stratigraphy of the Lower Paleozoic.