Most people are familiar with the human circulatory system, its heart, and its network of blood vessels (you should have learned it in health class!). It represents an example of a closed circulatory system, described as follows:
The blood always remains inside the blood vessels and never comes in direct contact with the cells. The materials enter and exit the blood vessels through the walls. The blood flows in the blood vessels under high pressure such that it reaches all the parts of the body in good time. The blood vessels are branched into fine capillaries which are actually involved in the exchange of materials.
But did you ever hear of an open circulatory system? Here is a nice description:
In higher animals, there are two primary types of circulatory systems — open and closed. Arthropods and mollusks have an open circulatory system. In this type of system, there is neither a true heart or capillaries as are found in humans. Instead of a heart there are blood vessels that act as pumps to force the blood along. Instead of capillaries, blood vessels join directly with open sinuses. “Blood,” actually a combination of blood and interstitial fluid called ‘hemolymph’, is forced from the blood vessels into large sinuses, where it actually baths the internal organs. Other vessels receive blood forced from these sinuses and conduct it back to the pumping vessels. It helps to imagine a bucket with two hoses coming out of it, these hoses connected to a squeeze bulb. As the bulb is squeezed, it forces the water along to the bucket. One hose will be shooting water into the bucket, the other is sucking water out of the bucket. Needless to say, this is a very inefficient system. Insects can get by with this type system because they have numerous openings in their bodies (spiracles) that allow the “blood” to come into contact with air.
And here is a picture to help you better visualize it:
So why bring this up?
Most of you are probably aware of the stunning convergence demonstrated by the cephalopod and vertebrate eye:
But this is not the only example of stunning convergence between the two groups. Most mollusks do indeed have an open circulatory system. But not cephalopods:
Cephalopods are the only mollusks with a closed circulatory system. They have two gill hearts (also known as branchial hearts) that move blood through the capillaries of the gills. A single systemic heart then pumps the oxygenated blood through the rest of the body.
Cephalopod circulatory system:
Human circulatory system:
So cephalopods an vertebrates not only show remarkable convergence when it comes to the eyes, but the circulatory system also shows the same thing.