Subphylum Trilobita
Subphylum Trilobita
Trilobites probably had their beginnings before the Cambrian period, in which they flourished. They have been extinct for 200 million years, but were abundant during the Cambrian and Ordovician periods. Their name refers to the trilobed shape of the body, caused by a pair of longitudinal grooves. They were bottom dwellers and probably scavengers (Figure 18-1A). Most of them could roll up like pill bugs, and they ranged from 2 to 67 cm in length.
Their exoskeleton contained chitin, strengthened in some areas by calcium carbonate. There were three tagmata in the body: head, thorax, and pygidium. Their head was one piece but showed signs of former segmentation; their thorax had a variable number of somites; and somites of the pygidium, at the posterior end, were fused into a plate. Their head bore a pair of antennae, compound eyes, mouth, and four pairs of jointed appendages. Each body somite except the last also bore a pair of biramous (two-branched) appendages. One of the branches had a fringe of filaments that may have served as gills.
Trilobites probably had their beginnings before the Cambrian period, in which they flourished. They have been extinct for 200 million years, but were abundant during the Cambrian and Ordovician periods. Their name refers to the trilobed shape of the body, caused by a pair of longitudinal grooves. They were bottom dwellers and probably scavengers (Figure 18-1A). Most of them could roll up like pill bugs, and they ranged from 2 to 67 cm in length.
Their exoskeleton contained chitin, strengthened in some areas by calcium carbonate. There were three tagmata in the body: head, thorax, and pygidium. Their head was one piece but showed signs of former segmentation; their thorax had a variable number of somites; and somites of the pygidium, at the posterior end, were fused into a plate. Their head bore a pair of antennae, compound eyes, mouth, and four pairs of jointed appendages. Each body somite except the last also bore a pair of biramous (two-branched) appendages. One of the branches had a fringe of filaments that may have served as gills.
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