What is characterized by the absence of blood vessels in its system?

An open circulatory system is characterized by the absence of blood vessels. In this type of circulatory system, blood (often termed hemolymph) is not enclosed within vessels like it is in closed circulatory systems. Instead, it flows freely through cavities or spaces within the organism's body, allowing for the direct exchange of nutrients and gases with tissues.

This system is typically found in invertebrates, such as arthropods and mollusks. The lack of blood vessels means that the movement of hemolymph is not as precisely controlled as in closed systems, which can lead to a less efficient distribution of nutrients and oxygen. Nonetheless, open circulatory systems can work effectively for smaller or less metabolically active organisms, where the demands for oxygen and nutrient delivery are lower.