Complex sentence: Hemocyanins carry oxygen in the blood of some arthropods such as the horseshoe crab, and most molluscs, and is second only to hemoglobin in biological popularity of use in oxygen transport.
Split into simpler sentences: Hemocyanins carry oxygen in the blood of most molluscs, and some arthropods such as the horseshoe crab. They are second only to hemoglobin in biological popularity of use in oxygen transport.