The term “worm” applies to an enormous variety of creatures across multiple phyla, exhibiting a wide spectrum of body plans and internal systems. However, most people asking this question are thinking specifically of the familiar earthworm, a segmented organism belonging to the phylum Annelida. This article will focus on the anatomy of the earthworm and contrast it with other major worm groups.
The Earthworm’s Circulatory System
The common earthworm possesses a closed circulatory system, meaning its blood remains contained within a network of vessels. Within the anterior segments, there are five pairs of structures often referred to as hearts, which are more accurately named aortic arches. These arches wrap around the earthworm’s esophagus.
The aortic arches connect the two main blood vessels that run the length of the body: the dorsal blood vessel and the ventral blood vessel. The muscular dorsal vessel pulses to push blood toward the head end of the worm. The ventral vessel then carries blood toward the tail end, supplying the body segments with nutrients and oxygen.
Function of the Aortic Arches
While the aortic arches perform the function of a heart, their structure is considerably simpler than those found in vertebrates. They are essentially muscularized blood vessels that contract rhythmically to regulate blood pressure and drive the flow from the dorsal to the ventral vessel. This contraction ensures consistent blood movement through the entire segmented body.
Unlike a human heart, the earthworm’s aortic arches are single-chambered and lack specialized chambers, complex valves, or intricate nerve bundles. Their purpose is to generate the necessary force to push the fluid through the extensive vessel network. This simpler design is appropriate for the earthworm’s body size and metabolic demands.
Circulation in Other Types of Worms
The segmented earthworm’s system is just one model for internal transport, and many other worms have entirely different mechanisms. For example, roundworms (phylum Nematoda) have no defined circulatory system. These animals rely on the fluid that fills their body cavity, known as the pseudocoelom, to circulate nutrients and waste.
The movement of the worm’s body facilitates the sloshing of this pseudocoelomic fluid, which is enough to distribute substances throughout their small, cylindrical bodies. Flatworms (phylum Platyhelminthes) have no specialized circulatory or respiratory organs. Their flattened body shape means that all cells are close to the exterior surface or the digestive cavity.
Flatworms rely entirely on the process of diffusion for gas exchange and nutrient distribution. The absence of a dedicated circulatory system restricts flatworms to small body sizes and flattened shapes to maintain this reliance on simple diffusion. Consequently, the answer to how many hearts a worm has can range from zero to ten, depending on the specific species being considered.