iBT TOEFL Topic Vocabulary Breakthrough Unit 066

iBT TOEFL Topic Vocabulary Breakthrough Unit 066
Author: Li Xiaolai

66. Insects are invertebrates, animals without backbones. They belong to a category of invertebrates called arthropods, which all have jointed legs, segmented bodies, and a hard outer covering called an exoskeleton. Two other well-known groups of arthropods are crustaceans, which include crayfish and crabs, and arachnids, which include spiders, ticks, mites, and scorpions. Many types of arthropods are commonly called bugs, but not every "bug" is an insect. Spiders, for example, are not insects, because they have eight legs and only two main body segments.

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