Echinoderms and Arthropods
Echinoderms include sea stars, sand dollars, and sea urchins. Despite their adult radial symmetry, many echinoderms begin life with bilateral symmetry before transforming as they mature.
The phylum Arthropoda (meaning "jointed feet") is the largest animal group on Earth. These creatures have segmented bodies with smaller sections fused into larger body regions. They're protected by an exoskeleton that provides support and protection but requires them to molt as they grow.
Arthropods are divided into several classes with distinct features. Arachnids (like spiders) have eight legs total. Crustaceans include shrimp, crabs, and lobsters, which must periodically shed their rigid exoskeletons to grow. Insects are distinguished by having exactly six legs - a key difference from arachnids.
Did You Know? The total weight of all ants on Earth is roughly equal to the total weight of all humans! Arthropods truly dominate our planet by numbers and biomass.