The question of whether a worm bleeds depends entirely on the type of worm encountered. While the term “worm” loosely describes thousands of species, including segmented earthworms, non-segmented roundworms, and flatworms, only some possess a true blood-like substance. For those that do, their circulatory fluid is chemically and structurally distinct from the red, cell-filled blood of vertebrates like humans.
Not All Worms Are Created Equal: Circulatory System Diversity
The ability to “bleed” is determined by the presence and type of circulatory system a worm has. Annelids, the phylum that includes earthworms and leeches, feature the most developed system, which is why they are the worms that most closely “bleed” when cut. Earthworms employ a closed circulatory system, meaning their blood is always contained within a network of vessels. These segmented worms use large vessels and five pairs of specialized muscular tubes, often called aortic arches, to pump the blood through the body.
Nematodes, or roundworms, lack any dedicated blood vessels or pumping structure. Instead, they rely on simple movements of their body to circulate nutrients and gases through the fluid-filled body cavity, known as a pseudocoelom.
Flatworms (Platyhelminthes), such as planarians, represent the simplest form and lack a circulatory system entirely. Their flattened body shape allows them to move oxygen, carbon dioxide, and nutrients through diffusion. Since they have no blood or vessels, there is nothing for them to bleed when injured.
The Chemistry of Worm Hemoglobin and Other Pigments
For the annelids that do have blood, its chemical composition differs from human blood. In earthworms, the oxygen-carrying protein, a form of hemoglobin, is not packaged inside red blood cells. Instead, this respiratory pigment is dissolved directly into the fluid portion, or plasma, of the blood.
This worm hemoglobin is an iron-based protein, which is why earthworm blood appears red when oxygenated, similar to human blood. The worm version is an enormous, multi-subunit protein complex that is much larger than its human counterpart. This size prevents the protein from leaking out of the blood vessels during circulation.
Some marine annelids, known as polychaetes, utilize a different pigment called chlorocruorin. This compound is also iron-based, but its slightly different chemical structure causes it to appear bright green when the blood is dilute or deoxygenated. When concentrated, it can take on a light red hue.
Other invertebrates, such as some mollusks and arthropods, use hemocyanin, which is a copper-based protein that turns blue when it binds oxygen. While less common in land worms, this pigment shows how non-vertebrate animals transport oxygen. The presence of these large, dissolved pigments means that worm “blood” is often more of a tinted plasma than a fluid full of cells.
Roles of Worm Circulatory Fluid
The fluid in a worm’s body, whether true blood or coelomic fluid, serves several functions beyond oxygen transport. It is responsible for moving digested food particles from the gut to the rest of the body. It also collects metabolic waste products to be processed and expelled by the excretory organs.
A primary function, particularly in earthworms, is providing a hydrostatic skeleton. The segments of the worm’s body are filled with incompressible fluid, and surrounding muscles contract against this fluid pressure. This high internal pressure allows the worm to stiffen sections of its body to push through soil or anchor itself.
By constricting the circular muscles, the worm forces the fluid to lengthen the body; contracting the longitudinal muscles shortens and thickens the body. This fluid-based skeletal system enables the earthworm’s characteristic burrowing and crawling movements. Finally, the fluid contains immune cells that function in defense against invading pathogens.