•• PHYLUM 8. JOINT-LEGGED ANIMALS, OR ARTHROPDA.
This group includes crustaceans, centipedes, spiders and the insects. Arthropods have three germ layers and two-sided symmetry. Their bodies are segmented, although the segmentation may be partly hidden. The body cavity has a big blood space. There is a digestive tube with mouth and anus. The nervous system is a double cord lying along the floor of the body, with a brain up front. There is usually a hard or tough body covering called an exoskeleton (outside skeleton). The legs and other appendages are jointed or hinged. The word Arthropoda means " hinged feet." By far the greater number of arthropods are insects. Insects are in many ways the most advanced of the invertebrates. Science-fiction writers like to imagine insects taking over the world, but man will very likely keep the insects in their place, because they have hardly any sense at all. There are 650,000 known species of arthropods.