Why is the way an organism gets oxygen not a characteristic of arthropods?

Answer 1

Arthropods do in fact have an aerobic metabolism requiring oxygen, though it differs from our own. The strategy to acquire it varies depending on environment; marine arthropods like crabs and lobsters have gills to extract it from water; crabs on land can still use gills if they remain moist. Insects, which constitute the bulk of the phylum, get it directly from the air through holes in their body segments (spiracles); the air gets conducted directly to the tissues via tubules and thus do not use a fluid circulatory system for that purpose. Those arthropods that do use their circulatory system for oxygenation exploit hemocyanin, a component in their body fluid (hemolymph) to which oxygen can bind, comparable to a mammal's hemoglobin. Unlike us, this oxygen carrying molecule is not bound to proteins in blood cells, but drifts around freely in the hemolymph.

The circulatory system for both those arthropods that oxygenate directly from air or oxygenate using hemolymph, is described as an "open" one, with the organs bathed in the fluid in a body cavity called a hemocoel.