Final answer:
Sea squirts are interesting because they digest their own cerebral ganglion during metamorphosis, reflecting a simpler circulatory system in contrast to the complex closed circulatory systems of cephalopods, like squids, which support high metabolism and rapid movement.
Step-by-step explanation:
What is interesting about blood flow in sea squirts is related to the changes they go through during their life cycle. Sea squirts start off as free-swimming larvae with a notochord and a basic nervous system, including a cerebral ganglion that operates similarly to a human brain. As they metamorphose into a sessile adult, they digest their cerebral ganglion, as they no longer need it for movement, leading to the saying that sea squirts 'eat their own brain'. In adult sea squirts, circulation is simpler and does not involve complex movement, contrasted with more active marine animals like squids which have evolved a closed circulatory system to rapidly move and catch prey.
The organization of the circulatory system in aquatic animals like fish shows a single circuit for blood flow with a two-chambered heart, limiting the amount of oxygen that reaches organs and tissues. In contrast, cephalopods, which include squids, have a more complex closed circulatory system due to their active lifestyles and need to move rapidly. Thus, the change in the circulatory system organization in cephalopods supports their more complex body designs and lifestyle compared to other mollusks and simpler sea creatures like sea squirts.