The largest group in the animal kingdom, invertebrates, include sponges, worms, mollusks, arthropods, and echinoderms.  They are multicellular, living on land and in water habitats. They range in size from microscopic dust mites to giant squids, have no spine, are heterotrophic, have radial or bilateral symmetry, are ectothermic, and mobile. We learn about the characteristics, habitats, systems, and body structures of each group. Terms: flagella, spicules, setae, annuli, tentacles, radial symmetry, bilateral symmetry, jointed appendages, exoskeletons, and regeneration. This diverse group is important in balancing earth's ecosystems.