Complete the crossword, then click on "Check" to check your answer. If you are stuck, you can click on "Hint" to get a free letter. Click on a number in the grid to see the clue or clues for that number.

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Across: 1. A group of animals characterized by a hard outer skeleton; includes millipedes, crustaceans, insects, and spiders. 3. An infectious particle consisting of nucleic acids and proteins. A ____ cannot reproduce on its own, and must instead use the cellular machinery of its host to reproduce. 5. A controlled manipulation of nature designed to test a hypothesis. 7. A characteristic of an organism that improves the organism's ability to survive and reproduce in its environment. 9. The process by which an organism grows from a single cell to its adult form. 14. A single-celled or multicellular organism in which each cell has a distinct nucleus and cytoplasm. 15. A single-celled organism that does not have a nucleus. All prokaryotes are members of the kingdoms Bacteria or Archaea. 16. A community of organisms, together with the physical environment in which the organisms live. 18. An association of populations of different species that live in the same area. 19. All living organisms on Earth, together with the environments in which they live. 20. Change over time in a lineage of organisms. Change in the genetic characteristics of populations over time. The history of the formation and extinction of species over time. 21. The largest taxonomic category in the Linnaean hierarchy. Generally six kingdoms are recognized: Bacteria, Archaea, Protista, Plantae, Fungi, and Animalia.

Down: 2. A possible explanation of how a natural phenomenon works. A hypothesis must have logical consequences that can be proved true or false. 4. Any of a group of six-legged arthropods that includes grasshoppers, beetles, ants, and butterflies; the most species-rich group of animals on Earth. 6. A kingdom of microscopic, single-celled organisms that was the first of the kingdoms to arise. 8. A major terrestrial or aquatic life zone, defined either by its vegetation (terrestrial biomes) or by the physical characteristics of the environment (aquatic biomes). 10. A group of interacting individuals of a single species located within a particular area. 11. A group of interbreeding natural populations that is reproductively isolated from other such groups. 12. A specialized reproductive structure that is characteristic of the plant group known as the angiosperms, or flowering plants. 13. A group of animals that have backbones. Vertebrates include fish, amphibians, mammals, birds, and reptiles. 17. The kingdom of mushroom-producing species, yeasts, and molds, which usually live as decomposers.