What term refers to invertebrate animals such as insects and spiders?

The term that refers to invertebrate animals such as insects and spiders is "Arthropod." Arthropods are characterized by their exoskeleton, segmented bodies, and jointed appendages, which include a wide variety of creatures such as insects, arachnids (spiders and scorpions), crustaceans (crabs and lobsters), and more. This diverse group is the largest phylum in the animal kingdom, showcasing an incredible range of forms and adaptations.

The other terms refer to different classifications of invertebrates. Segmented worms, or annelids, include earthworms and leeches, which have a body composed of segments but lack the jointed appendages of arthropods. Mollusks, such as snails and octopuses, have soft bodies and often a hard shell, differing significantly from the segmented and jointed nature of arthropods. Echinoderms, including starfish and sea urchins, possess a unique radial symmetry and a water vascular system, making them quite distinct from both arthropods and segmented worms. Understanding these distinctions helps to clarify the defining features of arthropods and their importance in ecological systems.