The Circulatory System of the Mollusca
The mollusca have an open circulatory system. In this type of circulatory system, heart or capillaries which are found in humans are only seen in some species. The blood in open circulatory systems is actually a combination of blood and interstitial fluid called hemolymph. The mollusca have blood vessels, which are similar to pumps, that push the blood throughout the body. These blood vessels are also directly connected to open sinuses. The blood is forced form the blood vessels into these large sinuses, which are in the internal organs, where an exchange of materials take place. Afterwards new vessels receive the blood from these sinuses and the movements of the body muscles gradually bring the hemolymph (blood) back to the sinuses. This circulatory system seems to be less efficient than a human's closed circulatory system but they both serve different purposes and have different levels of performance and the demands that the organism's body requires.
Octopus
Octopuses have a closed circulatory system. They have one main heart and two smaller hearts which are all crucial for maintaining the robust blood pressure that allows them to be active hunters and powerful swimmers. The large heart is called a systemic heart and the two smaller hearts are located near the gills. These two smaller hearts pump blood to the gills where it dumps the waste and picks up oxygen. Then the two smaller hearts pump the oxygen-filled blood towards the main heart which pumps the blood to the rest of the body. They also have blue blood because they don't have hemoglobin, like humans do, but instead they have a protein called hemocyanin.
Snail
The snail has an open circulatory system. The heart of a snail is located in the anterior part of the visceral mass. The snail has two chambers, an auricle and a ventricle. The auricle receives haemolymph from the lungs or gills and then ventricle pumps it into the aorta. Haemolymph is a mixture of blood and intestinal fluid, that is circulated throughout the snail, because the blood and intestinal fluid is sometimes indistinguishable. Some earlier snails have two gills supplying two separate auricles which is why the heart has 3 chambers. The aorta of the snail short and quickly divides into two main vessels, where one supplies the visceral mass and the other supplies the head and foot. These two main vessels divide into many smaller vessels and deliver the haemolymph to open arterial sinuses, where the oxygen is dropped off. Afterwards, the de-oxygenated blood flows into a large venous sinus inside the head and foot. The venous sinus contains nephridium, which is an excretory organ similar to a kidney. Then the haemolymph passes through the capillary network of the pulmonary lung and into the heart.
Oyster
Oysters have an open circulatory system. The ventricle, pericardium, intestine, and auricle are all vital parts for the open circulatory system of an oyster. At first, the blood is pumped out from the heart and enters a few large vessels and to the tissues where it exits into large spaces where it passes over the cells of various organs and drops off blood. Then this de-oxygenated blood drains into large areas called lacunae and into the kidneys and gills. From there, the blood returns to pick up oxygen and is pumped out through the heart and the cycle repeats.
- James Ikeda