Snails, belonging to the class Gastropoda, possess a heart that pumps circulatory fluid. While this function is similar to that of a human heart, the snail’s structure and system are fundamentally different from those of vertebrates. The snail’s circulatory system is adapted for a slower metabolism and lower-pressure circulation. It relies on a central pump to move fluid through a network that is not entirely closed.
Anatomy of the Mollusk Heart
The gastropod heart is a muscular organ typically housed in a thin-walled sac called the pericardium, situated in the anterior portion of the visceral mass. In most species, the heart features a simple two-chambered design, consisting of a receiving chamber called the auricle, or atrium, and a primary pumping chamber known as the ventricle. The auricle collects oxygenated fluid returning from the snail’s respiratory organ, which is either a gill in aquatic species or a lung-like structure in terrestrial species.
The ventricle is significantly more muscular than the auricle and is responsible for forcing the circulatory fluid out into the rest of the body. A set of semilunar valves guards the small opening between the auricle and the ventricle, ensuring that fluid flows in only one direction. The ventricle connects to a short, large vessel called the aorta, which quickly divides into major arteries that supply the head, foot, and visceral organs. The contractile tissue making up the heart muscle has been observed to contain striated fibers.
While the two-chambered heart is the most common arrangement, some primitive aquatic gastropods have a slight variation. These species may possess two separate gills, each supplying its own auricle, resulting in a three-chambered heart. Regardless of the number of chambers, the heart is strategically positioned near the respiratory surface to efficiently receive oxygenated fluid before distribution.
How the Open Circulatory System Works
The heart pumps fluid through an open circulatory system, which is a major distinction from the closed, high-pressure system found in mammals. The fluid, called hemolymph, is pumped from the ventricle into the aorta and then into smaller vessels that deliver it toward the tissues. These vessels do not branch into a dense network of fine capillaries; instead, they empty the hemolymph directly into open body spaces.
These open spaces are known as sinuses or lacunae, and they surround the internal organs, bathing them directly in the circulating fluid. This direct contact facilitates the exchange of gases, nutrients, and waste products between the hemolymph and the tissues.
After the exchange of materials occurs in the sinuses, the de-oxygenated hemolymph is collected and flows through larger venous sinuses. This fluid then passes through the excretory organ, the nephridium, which functions similarly to a kidney, before moving to the respiratory structure for re-oxygenation. From the respiratory organ, a vessel returns the freshly oxygenated hemolymph to the auricle, completing the cycle.
Hemocyanin: The Blue Blood
The circulatory fluid in a snail is hemolymph, which is distinct from vertebrate blood. Its respiratory pigment is hemocyanin, a copper-containing protein responsible for transporting oxygen throughout the body. Unlike the iron-containing hemoglobin found in human red blood cells, hemocyanin is copper-based.
When hemocyanin binds to oxygen, the copper at its core causes the hemolymph to take on a pale blue tint. This blue color is characteristic of most mollusks, though a few exceptions, like the freshwater Planorbid snails, use hemoglobin and thus have red hemolymph. A significant difference from human blood is that hemocyanin is not contained within cells but is instead dissolved directly in the hemolymph fluid.
Molluscan hemocyanin is one of the largest known proteins, forming massive, cylindrical structures highly efficient for oxygen transport in the snail’s low-pressure system. This copper-based pigment allows the hemolymph to effectively pick up oxygen at the gill or lung and release it to the tissues in the sinuses.