Final answer:
Open circulatory systems feature open-ended blood vessels where hemolymph, which is the fluid that serves as blood, circulates within a cavity called a hemocoel and directly bathes organs, unlike in closed circulatory systems where blood is contained within vessels.
Step-by-step explanation:
A characteristic of open circulatory systems but not closed circulatory systems is A) open-ended blood vessels. In an open circulatory system, the fluid that functions as blood, called hemolymph, is not fully enclosed within blood vessels. Instead, it is pumped into a body cavity called a hemocoel, where it directly bathes the organs. Hemolymph then returns to the heart through openings called ostia. This is distinct from a closed circulatory system where blood is always separated from the interstitial fluid and contained within a complete network of vessels, circulating unidirectionally from the heart to the body and back again.