Why are earthworms more evolved than pillbugs?
Pillbugs are isopods, which are crustaceans, in the Class Arthropoda. They arose approximately 300 million years ago. They are small and live in moist areas on land. They have seven pairs of legs, flattened, and can roll into a ball for protection. They have a chephalic shield covering their head, and have lung-like structures toward the rear of the body to breathe. To replace their exoskeleton, they molt in two phases when they grow. Their larvae hatch as mini-adults, except for the lack of the last pair of thoracic legs, and they don't have a swimming phase as a larvae. Earthworms are Oligochaetes, in the phylum Annelida. They possess a tube within a tube body plan, where the digestive tube is inside the body tube. This is considered an evolutionary advancement. They also have a simple closed circulatory system, whereas Arthropods have an open system. This is a more efficient method of transport within the body and their blood contains hemoglobin, a molecule that can readily absorb and transport oxygen throughout the body. Arthropods lack hemoglobin. Earthworms are hermaphrodites and can cross fertilize another worm. Their clitellum helps them form a cocoon to place the eggs in. Eventually, small, fully formed worms emerge and become adults within a year. Although both organisms are Invertebrates, earthworms have more complex body systems, than pillbugs.