The bodies of trilobites are divided into three parts (tagmata): a cephalon (head), composed of the two preoral and first four postoral segments completely fused together; a thorax composed of freely articulating segments; and a pygidium (tail) composed of the last few segments fused together with the telson. In the most primitive trilobites the pygidia are still fairly rudimentary. The thorax is fairly flexible - trilobites are often found curled up into balls, like modern sow bugs do for protection when disturbed.
Trilobites had a single pair of preoral antennaee and otherwise undifferentiated biramous limbs. Each leg had 8 segments (?), analogous to those of other arthropods, with no knee joint. The first segment also bore a featherlike epipodite, or gill branch, which was used for respiration and swimming. The limbs were covered by lateral projections called pleural lobes, extending outward from a central axial lobe; it is this tripartite division that gives trilobites their name.
Trilobites were only armored on top, but still had a fairly heavy exoskeleton. In molting it generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other: many trilobite fossils are actually molts, rather than dead individuals. In most groups there were two facial sutures on the cheeks to make shedding it easier. The cheeks usually also supported a pair of crescent-shaped compound eyes, which are often surprisingly advanced.
Trilobites had unique eyes, which were made of calcite (Calcium carbonate - CaCO3). Pure forms of calcite are transparent, and the trilobites used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The trilobite eyes were compound, with each lens being an elongated prism. The number of lenses in such an eye varies, however: some trilobites had only one, and some had thousands of lenses in one eye. In these compound eyes, the lenses are arranged in a hexagonal way (nature's way of close packing). Of course there's never a rule without an exception, and to prove that there were blind trilobites as well.
An egg hatched to give a tiny larva called a protaspid, in which only the fused segments of the cephalon are present. Subsequent thoracic segments were added behind the cephalon in successive molts in intermediate stages called meraspids, until finally the adult number of segments was reached, at which point the animal is called a holaspid. Trilobite larvae are reasonably well known and provide an important aid in their classification.