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Trilobite, extinct marine animal (phylum Arthropoda) from the Paleozoic Era (570–240 million years ago). Scientists have identified about 10,000 species from the fossilized remains of these small (4 in/10 cm) invertebrates. Their name (Latin, “3 lobes”) refers to the 3 sections created by 2 lengthwise grooves across their soft, horny shells. The thorax contained many sections, each bearing legs that had gills, enabling the trilobites to breathe.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Transcendentalism to United Church of Christ