Why molluscks have open circulatory system?

Mollusks have an open circulatory system primarily because it is simpler and more efficient for their body structure and lifestyle. In this system, blood is not confined entirely to vessels; instead, it flows freely through cavities, allowing for easier nutrient and gas exchange in their often less active bodies. This system is sufficient for many mollusks, given their relatively low metabolic rates compared to organisms with closed circulatory systems. Additionally, the open circulatory system reduces the energy needed for circulation, which is advantageous for these animals.