What animals typically have open circulatory systems?

Animals such as insects, spiders, and most molluscs typically have open circulatory systems.

Open circulatory systems are common in invertebrates, including insects, spiders, and most molluscs. This type of circulatory system is characterised by the blood, or haemolymph, flowing freely within the body cavity, or haemocoel, and directly bathing the organs and tissues. The heart pumps the haemolymph through arteries, but unlike in closed circulatory systems, there are no veins to return the fluid to the heart. Instead, the haemolymph circulates back to the heart through open spaces.

Insects, for instance, have a tubular heart that runs along the length of their body. The heart pumps the haemolymph into the aorta, which then empties into the body cavity. The haemolymph circulates around the organs, delivering nutrients and removing waste products. It then returns to the heart through openings called ostia.

Similarly, spiders have an open circulatory system with a simple tubular heart and arteries that deliver haemolymph to their body cavity. The haemolymph then returns to the heart through ostia.

Most molluscs, including snails, clams, and squid, also have open circulatory systems. However, cephalopods like octopuses and squids are exceptions, as they have closed circulatory systems. In molluscs with open circulatory systems, the heart pumps haemolymph into the haemocoel, where it bathes the organs directly. The haemolymph then returns to the heart through ostia.

In summary, open circulatory systems are a common feature in many invertebrates, including insects, spiders, and most molluscs. This system allows for the direct exchange of nutrients and waste products between the haemolymph and the body's organs and tissues.