Earthworms are tube-shaped segmented organisms that fall under the phylum Annelida. Contrary to popular perception, earthworms are not insects or arthropods—they are animals. Earthworms have a widely varied diet and will feed on plants, leaves, fruits, berries, vegetables, fungi, algae and microorganisms.
Earthworm excretions are very rich in nitrogen and phosphates, both of which are good for healthy soil and plant growth. As such, earthworm activity is seen a very beneficial to farmers, and earthworm activity plays an important biological role in the breakdown and cycling of organic material.
Earthworms exist on every continent on the planet except Antarctica, and they are numerous and commonly found wherever they live. The ecological benefits of earthworms have been known for a long time as the Greek philosopher Aristotle called earthworms the “intestines of the Earth.” The naturalist Charles Darwin, in addition to his theory of evolution by natural selection, was well-known for his observations of earthworm behavior, and once wrote that “it may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised [sic] creatures.”
Phylogenetic analysis suggests that the most recent common ancestor of all living species of earthworm emerged approximately 209 million years ago, meaning that earthworms are about as old as the first mammals and some species of dinosaur. The evolutionary trajectory of earthworms seems to be split amongst two major clades, with a majority of earthworm species falling under one clade in the Lumbricidae and Megascolecoidea families. Earthworms likely spread all over the world via a combination of dispersal between continents due to the activity of other animals and as a result of continental drift over geological ages.
The typical earthworm grows to be about 14 inches long, but species of earthworm range from a minuscule 10mm long to a whopping 10 ft long (Amynthas mekongianus). The earthworm body is segmented with about 50-150 individual segments (called metamerisms) on its body. Pores situated along these segments secrete a slippery fluid that decreases friction and allows the worm to breathe.
Each segment of the body, except for the mouth and tail, are lined with tiny bristles called setae that the worms use to grasp and anchor its body during movement. Earthworm’s move through the tightly synchronized motion of two pairs of muscles along the body. One pair situated laterally will contract to squeeze and lengthen each segment. then another pair situated longitudinally will contract to pull and contract each segment, pulling the worm along the ground. The speed at which an earthworm moves is heavily dependent on the species and surface. Some earthworms can move relatively quickly and could cover over 70 meters in an hour while others move at a leisurely pace of 7 meters an hour.
Despite their simple appearance, earthworms have complex internal anatomy with several well-defined organ systems. Unlike many invertebrates, earthworms have a closed circulatory system, complete with well-defined vessels and even 5 pairs heart-like organs known as the aortic arches. The aortic arches located about halfway up the body, placed around the central body cavity known as the coelom. The aortic arches play the same functional role as the heart in humans; their contractions pump blood, fluid, and waste through the body. So in a sense, an earthworm actually has 5 pairs or 10 individual hearts! In total, earthworms have 5 major vessels each of which carries fluids to and from important parts of the body.
Earthworms have a central nervous system composed of a central brain and a ventral nerve cord analogous to a spinal cord in vertebrates. The earthworm brain is a small pear-shaped pair of cerebral ganglia that is located between the third and fourth segment from the mouth. The arrangement of the brain and subpharyngeal ganglia forms a circle around the pharynx. The CNS is what controls the motion, respiration, and digestion of the worm.
The earthworm CNS feeds into the organism’s peripheral nervous system, which consists of various nerve endings that serve to connect distal areas of the body to the brain. The PNS is composed of 8 to 10 individual nerves that stem directly from the brain along with 3 pairs of nerves that stem from the subpharyngeal ganglia. The peripheral nervous system allows the worm to respond to heat and pressure. Earthworms are thigmotactic, meaning that they respond to contact and touch, so the PNS mediates an important sensory modality.
Earthworms do not have any specialized respiratory organ like lungs. Instead, they breathe via the exchange of gas through their skin. Atmospheric oxygen will diffuse across the earthworm’s skin and enter the blood vessels, while carbon dioxide and other gaseous waste products diffuse out of the blood int the environment. The fluid secreted on the skin of an earthworm helps facilitate this diffusion and a lack of this moisture can prevent diffusion and suffocate the earthworm.
Unlike the tightly coiled intestines of humans, the gastrointestinal tract of an earthworm is just a straight line that runs the length of the body from mouth to anus. food entering the mouth is drawn into the esophagus by the contractions of the pharynx, where calcium is pumped in to regulate pH levels. Further, in the intestines, strong muscles of the coelom grind the food up and various enzymes are secreted by the intestinal wall to break down sugars, fats, and cellulose. Several folds of intestinal tract line the walls to increase the surface area and thus nutritive absorption of the digestive tract.
All earthworms share the same general body plan and lifecycle, but several have unique physical traits or behaviors that distinguish them from others. For instance, some earthworms are bioluminescent and will glow in the dark, some are capable of climbing up sheer rock surfaces, and others are bright and exotically colored.
Depending on the species, earthworm life expectancy can differ. Most common garden species only live for 1 to 2 year while heartier species can live an estimated 6 years. Earthworms do not go through distinct stages of specific development. A newborn earthworm is morphologically similar to an adult earthworm except they lack sex organs. Newborn earthworms develop their reproductive organs 60 to 90 days after hatching.
All earthworms are hermaphroditic meaning that they possess both male and female sexual organs (ova and testes). Copulation and reproduction are actually two separate processes for earthworms. During copulation, each earthworm will overlap their front ends ventrally and exchange sperm. Each worm will store the other’s sperm until the time comes for them to fertilize their eggs. As an earthworm reaches sexual maturity, their clitellum will swell and change color to become very pinkish red. After copulation, the clitellum will secrete mucus that forms a ring around the worm’s body. The earthworm will back out of the ring, injecting its own eggs and the other worm’s sperm to create an onion shaped cocoon that incubates the worm embryos.
Incidentally, many species of earthworm are parthenogenetic, meaning that they can stimulate the growth of new embryos from eggs without fertilization. In the absence of mating partners, worms will undergo parthenogenesis to reproduce, creating an identical female clone. Most species of earthworm capable of parthenogenesis are also capable of sexual reproduction.
Ecology Of Earthworms
Earthworms play many vital roles in ecosystems as they are one of the main mechanisms by which organic matter is broken down and reintroduced into the soil.
Earthworms subsist on a diet of decaying organic matter, including fruits, vegetables, leaves, plant matter, and animal excrement. While burrowing, earthworms in the soil ingest minute sand and rock particles, that grind the organic material in the worm’s stomach into a fine paste which is then excreted in the soil. The grinding of the soil into a paste release the constituent minerals and nutrient into an accessible form that plants can use. Soil that has been treated with earthworms has been shown to have up to 5 times more nitrogen, 7 more phosphates, and 11 times of potassium than non-earthworm treated soil.
Additionally, the burrowing action of earthworms serves to aerate and drain soil. Earthworm burrows create crisscrossing channels through the ground that give air and water a way to be spread and drain out of the soil. The burrowing activity of earthworms has been likened to the action to a piston that pumps air into an engine.
The soil benefits of earthworms are so well-known that entire industries exist for the purpose of raising earthworms for agricultural use. It is estimated that each square meter of arable soil contains anywhere from 62 worms for poor soil to 432 worms for rich soil. The sustainability of earthworm activity has made them a favorite amongst permaculture enthusiasts.
Earthworms are also an important source of food in ecosystems. They serve as prey for animals of pretty much every major class of land-dwelling animals such as birds, mammals, reptiles, amphibians, insects, and arthropods. In many countries, earthworms are regularly consumed by humans and are even considered a delicacy in some places.
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