The Arthropoda include such familiar forms as insects, spiders, scorpions, millipedes, crabs, lobsters, shrimp, and a number of extinct groups, including the trilobites. These comprise the most diverse phylum on our planet, and there are well over one million described species. A large proportion of arthopods are insects, and some researchers have estimated that the total number of living arthropods may well exceed 30 million species. Arthropods are characterized by the possession of a jointed cuticular exoskeleton that contains chitin and proteins and sometimes other minerals such as calcium carbonate to make it harder. Because the skeleton is rigid and completely surrounds the internal tissues, arthropods must periodically shed their skeleton as they grow and produce a new one in a process known as molting (ecdysis). They have a segmented, bilaterally symmetrical body and a segmented exoskeleton as well. Many of the variations in the arthropod body plan involved the fusion of segments and the modification and specialization of the jointed appendages. Most arthropods have well-developed eyes. Arthropods can be found in a broad range of marine, freshwater, and terrestrial environments. The fossil record of the arthropods extends back to the Cambrian Period.
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Trilobites (Phylum Arthropoda, Class Trilobita)
Trilobites are extinct marine arthropods that were important components of marine communities during much of the Paleozoic Era. Trilobites had a hard, calcified, and segmented exoskeleton and are often well-preserved as fossils. Although parts of their exoskeleton, such as the legs and antennae, were not mineralized, there are rare fossils with soft parts preserved that clearly show that their body segments each had a pair of jointed legs. Many trilobites had well-developed compound eyes. The name Trilobite stems from the division of the body into three longitudinal parts: a central (axial) lobe and two adjacent (left and right) pleural lobes. The body is also characterized by three major regions from front to back: the head, or cephalon, the segmented thorax, and the tail-like pygidium. Trilobites first appeared during the Cambrian Period and underwent a relatively rapid diversification in the both number of species and major morphological types during the Cambrian and Ordovician Periods. They were among the most diverse marine invertebrates during much of the Paleozoic Era, and over 17,000 species have been described. Most trilobites were probably scavengers and detritvores moving about on the sea floor, although many different life-habits evolved within the group, including forms that were predators and suspension feeders. Major losses in trilobite diversity occurred during the Devonian Period, and only one group (Order Proetidae) survived beyond the Devonian. Trilobites eventually disappeared during the great extinction at the end of the Paleozoic Era (~250 mya).
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