Final answer:
Hemoglobin is better for phoronids due to its higher oxygen-carrying capacity compared to other pigments like hemocyanin and hemerythrin. Its efficiency is crucial for animals with high metabolic rates and is important in environments with limited oxygen availability.
Step-by-step explanation:
Hemoglobin is the best bet for phoronids because it has a higher oxygen-carrying capacity compared to other respiratory pigments like hemocyanin, chlorocruorin, and hemerythrin. Hemoglobin, a heme protein, is composed of four protein subunits and a heme group that has iron associated with it. The iron in hemoglobin reversibly associates with oxygen, facilitating efficient oxygen transport. In contrast, other pigments like hemocyanin, which is found in mollusks and some arthropods, use copper instead of iron, leading to a lower oxygen-carrying capacity. Hemerythrin, though iron-containing, does not contain a heme group and is similarly less efficient in oxygen transport.
Moreover, for multicellular organisms, including phoronids, a higher oxygen demand necessitates a more effective transport mechanism, which hemoglobin provides. Additionally, in environments where oxygen availability is a limiting factor, hemoglobin's efficiency becomes even more critical. As such, hemoglobin is a superior molecule for oxygen transport in animals with higher metabolic rates and oxygen requirements.