How Many Hearts Do Earthworms Have?

The earthworm is an oligochaete annelid, which is distinguished by well-developed segmentation, the absence of parapodia (leg-like appendages), and a rounded head segment without sensory structures. In other words, the body of earthworms is quite peculiar, with an elongated shape divided into segments formed by groups of small rings, each of which has a specific function. Its shape allows it to crawl smoothly above and below the ground.

To supply this elongated body with nutrients and blood, a complex circulatory system is required, which, if we compare it to the human body, we could say it has several hearts.

Compared to other invertebrates, the circulatory system of worms is closed, as blood flows within blood vessels and does not seep into sinuses or lacunae. Basically, this system consists of different types of vessels:

• Dorsal vessel
• Ventral vessel
• Subneural vessel
• A series of aortic arches (commonly referred to as hearts)

These hearts, actually called aortic arches, pump the necessary blood to the two groups of blood vessels that supply the worm's body. In this way they divide their function equally between the first and second halves of the organism, efficiently reaching all its structures.

An earthworm can eat up to 90% of its body weight per day. Therefore, it is important to have an efficient system to cleanse the blood of waste products and eliminate them.

The number of hearts varies from species to species. For example, in the genus Lumbricus there are 5 hearts, while the genus Tubiflex has only 2 hearts.

The vessels or "hearts" have certain characteristics that distinguish them from the hearts of other animals. Here is a list of some of them:

• The blood vessels run longitudinally through the body of the worm and are connected by transverse vessels.
  
  
• The blood vessels have no endothelium and are lined only by the basal lamina of the peritoneum.
  
  
• The dorsal vessel or "true heart" is contractile, has valves, and is the main means of propelling the blood. It receives blood from the vessels of the body wall and digestive tract and propels it forward into five pairs of aortic arches.
  
  
• The ventral vessel is also unique. Unlike the dorsal vessel, it is noncontractile and functions as the aortic artery. It receives blood from the aortic arches and sends it to the brain and the rest of the body.
  
  
• The lateral vessels at the level of the esophagus are larger in caliber and volume and are also called hearts. They also have a contractile function to propel blood and keep blood pressure stable in the ventral vessel.

They are very beneficial to the soil

Earthworms eat leaves, compost, and other dead plant material to build their burrows. In this way they influence the physical properties of the soil, the nutrient cycle and the growth of plant organisms, regulating the dynamics of organic matter and the quality of the soil. Likewise, they facilitate and promote the work of microorganisms, since they accelerate the decomposition of dead matter and its mineralization by reducing the size of the particles. On the other hand, by improving the structure and quality of the soil, they increase the productivity of plant communities and influence their structure.

Ther are very voracious animals

Earthworms are very voracious animals, consuming more than 90% of their own weight in a day. Of this, they excrete between 50 and 60% again, which they incorporate into the soil, forming an excellent quality substrate: Worm humus. Read on to learn more about other organic fertilizers.

They breathe through the skin

Gas exchange occurs through the skin in most oligochaetes, including worms. The larger specimens usually have dissolved hemoglobin in the blood plasma.

They possess a reproductive structure called clitellum

The clitellum constitutes a ring surrounding the body of the worm. The glands present in this structure secrete mucilage for copulation, the cocoon wall, and the albumen on which the eggs are laid.

They are hermaphrodite

Worms have differentiated gonads, and in terrestrial animals there is frequently an ovarian segment followed by one or two testes. The female segments are usually located behind the male segments, and their location varies with the species.

They reproduce through sperm transfer

Reproduction occurs by sperm transfer when the anterior surfaces of the two worms touch. The anterior part of one is placed on the posterior part of the other. The mucilaginous covering of the clitellum helps them stay in this position.

Learn more about other animals that breathe through their skin in this other article, where we explain what cutaneous respiration is.

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