Snails, as members of the mollusk phylum, possess a heart. This muscular pump circulates a fluid called hemolymph throughout the body. While the heart’s function is similar to that of vertebrates, its structure and the system it supports are fundamentally different, adapted to the snail’s unique anatomy and lifestyle.
The Open Circulatory System
Snails utilize an open circulatory system, contrasting with the closed systems found in mammals and birds where blood remains confined within vessels. In an open system, the heart pumps hemolymph into short arteries that open into spaces within the body cavity called the hemocoel.
Within the hemocoel, the hemolymph directly bathes the organs and tissues, facilitating the exchange of oxygen, nutrients, and waste products. This free movement of fluid around internal organs defines the system as open. After exchange, the deoxygenated hemolymph is collected into large sinuses and flows back to the heart to complete the cycle. This less pressurized system is sufficient for a slow-moving organism like a snail.
The Snail’s Heart Structure
The snail’s heart is a muscular, two-chambered organ typically situated within the pericardial cavity. It consists of one auricle (atrium) and one ventricle. The auricle is the receiving chamber, collecting oxygenated hemolymph returning from respiratory organs like the gill or lung.
The hemolymph flows from the auricle into the ventricle, which serves as the stronger, main pumping chamber. Valves are often present between the auricle and the ventricle to prevent backflow during contraction. The ventricle then pumps the hemolymph into the aorta, a short main artery that rapidly divides to distribute the fluid to the body’s tissues and the hemocoel.
Hemolymph: The Snail’s Blood
The circulatory fluid in a snail is called hemolymph. Its primary respiratory pigment is hemocyanin, a large protein that binds oxygen. Unlike the iron-based hemoglobin in human blood, hemocyanin uses copper atoms to facilitate oxygen transport.
When oxygenated, the hemolymph appears pale blue, though it is typically colorless when deoxygenated. Beyond transporting oxygen and nutrients, hemolymph functions as a hydrostatic skeleton. By regulating fluid pressure within the hemocoel, it helps the snail extend its foot for movement and retract its body into its shell. The hemolymph also aids in innate immunity and is involved in waste filtration before returning to the heart.