Cardiovascular system
A great pond snail's circulation is an open system, as opposed to the closed systems found in other animals. Snails have an aorta, which is the heart's major artery, and a pulmonary vein, which leads from the lungs to a snail's heart. However, because it is an open system, blood will flow between organs instead of through blood vessels. This blood mixes with lymphatic fluid to create hemolymph, which is the circulatory fluid of some invertebrates. The pond snail's heart is two chambered, meaning it has one ventricle and one atrium. The heart is housed in the pericardium, which is sometimes known as the "heart bag". The heart bag is also included as part of the snail's excretory system, as it filters out indigestible material through the heart bags' wall.