Animal Biology: Diversity, Form and Function
What you must understand
Animal biology tests classification together with function. Know the major invertebrate groups (porifera, cnidaria, platyhelminthes, annelida, mollusca, arthropoda, echinodermata) and vertebrates (fish, amphibians, reptiles, birds, mammals) by key features: symmetry (radial vs bilateral), body cavity, segmentation, exoskeleton vs endoskeleton, and thermoregulation (ectotherm vs endotherm). Arthropods are the most species-rich group; insects have three body regions and six legs.
Function follows environment. Gas exchange structures scale with body plan: gills in water, lungs and tracheae on land. Circulatory systems range from open (arthropods, most molluscs) to closed (annelids, vertebrates). Nervous systems range from nerve nets to centralised brains. Reproduction may be external (many fish and amphibians) or internal; development may be direct or via metamorphosis.
Traps
"Cold-blooded" does not mean the animal's blood is always cold; ectotherms rely on external heat, endotherms on metabolic heat. Amphibians are not reptiles: their skin is permeable and they typically need water for reproduction. Not all insects are bugs, and not all arthropods are insects (spiders have eight legs, two body regions). Bilateral symmetry does not imply a coelom. Open circulation still delivers nutrients - it is not "no circulation".
Worked example
Classify an animal with bilateral symmetry, segmentation, an exoskeleton, jointed legs and tracheal tubes. Segmentation plus jointed exoskeleton points to arthropoda; tracheae and three body regions point to insects. Rule: classify by the most specific shared derived feature, not by habitat.