Animals with an open circulatory system possess a haemocoel. Option 1, coelenteron, refers to the body cavity of cnidarians and is not related to circulatory systems. Option 2, spongocoel, is the central cavity in a sponge, which also does not pertain to circulatory systems. Option 3, pseudocoel, is a body cavity found in certain invertebrates, but it is not directly linked to circulatory functions. The haemocoel, mentioned in option 4 and confirmed as the correct answer, is a blood-filled body cavity found in animals with an open circulatory system. In these animals, the heart pumps blood into the haemocoel, where it directly bathes the organs and tissues. This is in contrast to animals with a closed circulatory system, where blood is circulated through a system of vessels.