Which groups of arthropods typically have two body regions, the cephalothorax and abdomen?

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Multiple Choice

Which groups of arthropods typically have two body regions, the cephalothorax and abdomen?

Two main body regions, a fused cephalothorax and a separate abdomen, show how some arthropods are organized. In arachnids, the head and thorax merge to form the cephalothorax while the abdomen sits behind it, as seen in spiders, scorpions, and their relatives. In many crustaceans, the same idea occurs: a cephalothorax created by a fusion of head and thorax (often with a protective carapace) and a distinct abdomen behind it, such as in crabs, lobsters, and shrimp. Insects, by contrast, have three distinct regions—head, thorax, and abdomen. Millipedes and centipedes have a head and a long trunk instead of a true cephalothorax, so they don’t fit the two-region pattern. Mollusks and echinoderms aren’t arthropods and follow different body plans. Therefore, the groups that typically exhibit two body regions (cephalothorax and abdomen) are crustaceans and arachnids.

Two main body regions, a fused cephalothorax and a separate abdomen, show how some arthropods are organized. In arachnids, the head and thorax merge to form the cephalothorax while the abdomen sits behind it, as seen in spiders, scorpions, and their relatives. In many crustaceans, the same idea occurs: a cephalothorax created by a fusion of head and thorax (often with a protective carapace) and a distinct abdomen behind it, such as in crabs, lobsters, and shrimp. Insects, by contrast, have three distinct regions—head, thorax, and abdomen. Millipedes and centipedes have a head and a long trunk instead of a true cephalothorax, so they don’t fit the two-region pattern. Mollusks and echinoderms aren’t arthropods and follow different body plans. Therefore, the groups that typically exhibit two body regions (cephalothorax and abdomen) are crustaceans and arachnids.

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