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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 pygidiumThe pygidium is the posterior body part or shield of crustaceans, some insects, and trilobites. It contains the anus and, in females, the ovipositor. It is composed of fused body segments, sometimes with a tail, and separated from thoracic segments by an (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 like modern sow bugs for protection.

Trilobite above Mesquite Springs in Death ValleyDeath Valley is a deep arid basin in the northern Mojave Desert of southern California in the United States, extending for approximately 140 mi (225 km) along the California-Nevada border approximatley 100 mi (160 km) west of Las Vegas. Famous for its bru

Trilobites had a single pair of preoral antennaAntennae (singular antenna , are the paired appendages connecting to the first (and in crustaceans also to the second) segment of the head of the members of all subphyla of the arthropods except Chelicerata. Also the members of the order protura lack theme and otherwise undifferentiated biramous limbs. Each exopodite (walking leg) had 6 segments, analogous to those of other early arthropods. The first segment also bore a featherlike epipodite, or gillGill In aquatic organisms In aquatic organisms, gills are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. Many small aquatic animals absorb oxygen through the surface of their bodies in general, but mor 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 exoskeletonExoskeleton in contrast of endoskeleton, is a general term to describe various external anatomical features that support and protect animals' bodies. Many invertebrate animals such as insects, crustaceans and shellfish have exoskeletons. Without scales, s. 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 easier. The cheeks usually also supported a pair of crescent-shaped compound eyes, which were surprisingly advanced in some species.

Trilobites had unique eyes, which were made of calciteThe mineral calcite is a carbonate of calcium corresponding to the formula CaCO and is one of the most widely distributed minerals on the Earth's surface. It is a common constituent of sedimentary rocks, limestone in particular. It also occurs as a vein m ( calcium carbonateProperties General Name Calcium carbonate Chemical formula Ca CO Appearance White solid Physical Formula weight 100. 1 amu Melting point Liquifies under high pressure at 1612 K (1339 °C) Boiling point decomposes at 1172 K (899 °C) Density 2. 7 ×103 kg/ m3, 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 typically 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 hexagonally (nature's way of close packing). Some trilobites were, however, blind, especially those species living too deep in the sea for light to reach them.

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 during an intermediate stage called meraspid, 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.

Trilobites appear to have been exclusively marine organisms since the fossilized remains of trilobites are always found in rock containing fossils of other salt-water animals such as brachiopods, crinoids, and coral, and they are found in a range of environments from extremely shallow water to very deep water. The tracks left behind by trilobites crawling on the sea floor are occasionally preserved as trace fossils.

Some trilobites evolved into elaborate spiny forms, particularly during the Devonian period. (For example, such specimens are found in the Devonian Hamar Laghdad formation of Alnif in Morocco).

Trilobites range in length from one millimetre to 70 cm (1/25 inch to 20 inches), with a typical size range of two to seven centimetres (1 to 3 1/2 inches).

The exact reason for the extinction of the trilobites is not clear, although it would seem to be no coincidence that their numbers began to decrease with the arrival of the first sharks and other early fishes in the Silurian and Devonian periods. Trilobites may have provided a rich source of food for these new arrivals. Their relatively low numbers and diversity at the end of the Permian no doubt contributed to their extinction during that great mass extinction event.

Trilobite fossils are found worldwide, with many thousands of known species. Because they evolved rapidly, trilobites serve as excellent index fossils, enabling geologists to date the age of the rocks in which they are found. They were among the first fossils to attract widespread attention, and new species are being discovered every year. The Native Americans had a name for trilobite which means "little water bug in the rocks"—a name which demonstrates extraordinary zoological acuity.

A famous location for trilobite fossils in the United Kingdom is Wren's Nest , Dudley in the West Midlands, where Calymene blumenbachi is found in the Silurian Wenlock Limestone formation. This trilobite is featured on the town's coat of arms and was named the 'Dudley locust' or 'Dudley bug' by quarrymen who once worked many of the now abandoned limestone quarries. The other trilobites found here include Dalmanites, Trimerus and Bumastus.