esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. The largest known arthropod, the sea scorpion Jaekelopterus rhenaniae, has been found in estuarine strata from about 390 Ma. It was up to 2.5 m (8.2 ft) long. Fossil trilobite. Trilobites first appeared about 521 Ma. They were highly successful and were found everywhere in the ocean for 270 Ma. Arthropods have an exoskeleton , a segmented body, and jointed appendages . They form a phylum which includes insects, arachnids, myriapods, and crustaceans. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments. Molluscs have such diverse shapes that many textbooks base their
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