Invertebrates are unique living things that help maintain balance in the natural world. Grade 2 PYP Sharing the Planet. 20 Vocabulary words with a picture, definition and an example sentence. The vocabulary words include: Invertebrates, Vertebrates, Characteristics, Behaviours, Adaptation, Interact, Impact, Habitat, Coexist, System, Cycle, Classify, Observe, Research, Label, Parts of, Unique, Mimiscule, Responsibility, and Respect.