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1. Animals are defined as heterotrophic organisms, meaning that they can only obtain their nutrients by eating.
2. Nematodes and arthropods
3. Flatworms’ flatness creates a high surface-area-to-volume ratio. This allows oxygen to easily diffuse from the environment and permeate their tissues. Therefore, they do not need a circulatory system to transport oxygen around their bodies.
4. Based on the phylogenetic tree, the phylum that includes echinoderms/sea stars is the closest relative of our phylum, the chordates.
5. True.
6. Nervous tissue and muscle tissue.
7. They have radial symmetry and, instead of a brain, they possess a nerve net.
8. Arthropods are the most numerous (it includes all insects!).