Most arthropods undergo metamorphosis (complete or incomplete). They have exoskeletons, open circulatory systems, and no notochord.
The muscular foot in molluscs like snails and clams is adapted for locomotion (in gastropods) and attachment to substrates (in bivalves).
The diaphragm is a muscular partition found only in mammals that aids in breathing. Other vertebrate classes use different mechanisms for respiration.
Frogs undergo indirect development with distinct larval stages (tadpole), while reptiles and birds show direct development without free-living larval forms.
Amphibians have a three-chambered heart with two atria and one undivided ventricle. Some mixing of oxygenated and deoxygenated blood occurs.
The production of milk through mammary glands is the defining characteristic of mammals (Class Mammalia). While other features may be shared, lactation is unique to mammals.
Flatworms (Platyhelminthes) use flame cells (protonephridia) for osmoregulation and excretion. Nephridia are found in annelids, Malpighian tubules in insects.
Insects and other arthropods have an open circulatory system where hemolymph bathes the organs directly. Vertebrates have closed circulatory systems.
Cnidarians (hydra, jellyfish, corals) have tentacles, radial symmetry, and nematocysts, but do NOT have a segmented body. Segmentation is characteristic of Annelida and Arthropoda.
The gastrodermis (or endoderm) is the inner layer of sponges that lines the spongocoel and contains choanocytes for filter feeding.