- Sponges are the most primitive of animal groups. They live in water (usually saltwater), are sessile (do not move from place to place), and filter tiny organisms out of the water for food.
- Coelenterates are also very primitive. Their mouths, which take in food and get rid of waste, are surrounded by stinging tentacles. Some coelenterates are jellyfish, corals, and sea anemones.
- Echinoderms include starfish, sea urchins, and sea cucumbers. They live in seawater and have external skeletons.
- Worms come in many varieties and live in all sorts of habitats — from the bottom of the ocean to the inside of other animals. They include flatworms (flukes), roundworms (hookworms), segmented worms (earthworms), and rotifers (philodina).
- Mollusks are soft-bodied animals, which often live in hard shells. They include snails, slugs, octopus, squid, mussels, oysters, clams, scallops, chitons, and cuttlefish. Mollusks are the second-largest group of invertebrates, with 50,000 living species.
- Arthropods are the largest and most diverse of all animal groups. They have segmented bodies supported by a hard external skeleton (or exoskeleton). Arthropods include insects, arachnids (spiders and their relatives), centipedes, millipedes, and crustaceans like crabs, lobsters, and shrimp.