Earthworms, as they are susceptible to pH, waterlogging, compaction, rotation, tillage and organic matter, are considered a good biological indicator of soil health. Their numbers and distribution in a field will indicate what is happening under the surface. In nearly all soils in the world, earthworms occur where the moisture and organic content are sufficient to support them.
The earthworm scientific name is Lumbricina.
There are more than 1,800 species of the Oligochaeta class of terrestrial worms. Members of the Lumbricus genus, in particular. The most common species of an earthworm is Lumbricus terrestris.
Currently, as given in a species name database, there are over 6,000 terrestrial earthworm species. Just about 150 species are widely distributed around the world, out of a total of about 6,000 species. These are earthworms, either peregrine or cosmopolitan.
Few of the common earthworm species are listed below. The name in the bracket is an earthworm scientific name.
Redhead Worm (Lumbricus rubellus)
Common Earthworm (Lumbricus terrestris)
Green Worm (Allolobophora chlorotica)
European Nightcrawler (Eisenia hortensis)
Brandling Worm (Eisenia fetida)
Giant Gippsland Earthworm (Megascolides australis)
Kentucky Earthworm (Komarekiona eatoni)
Oregon Giant Earthworm (Driloleirus macelfreshi)
Louisiana Mud Worm (Lutodrilus multivesiculatus)
Washington Giant Earthworm (Driloleirus americanus)
Gray Worm (Aporrectodea calignosa)
African Nightcrawler (Eudrilus eugeniae)
Composting Worm (Perionyx excavatus)
Morphology of Earthworm
Shape and size of an earthworm
Earthworms are generally broad, small, cylindrically elongated with points at the front, blunt behind, and thickest slightly behind the anterior end.
They are Bilaterally symmetrical.
The presence of a dark median line of a dorsal blood vessel that runs just below the skin in the body is indicated by the dorsal surface of the body.
The presence of genital openings and papillae in the anterior sections of the body marks the ventral surface.
Size varies from species to species and from people to people of the same species.
A mature earthworm has a length of around 150 mm and a width of 3 to 5 mm.
The structure of earthworm is given in the below diagram. The main parts of an earthworm are as explained as follows:
The mouth is a crescentic anterior aperture. On the ventral line, it is located just below the prostomium. Surrounded by the peristomium or buccal segment of the 1st segment of the body.
The exit of the alimentary canal is anus which is a vertical slit-like aperture at the posterior terminus. Undigested wastes are removed from it.
The earthworm is a hermaphrodite, but in the same individuals, male and female generative openings are found.
There are 4 pairs of small ventrolateral spermathecal pores which lie intersegmental between the grooves of 5/6, 6/7, 7/8, and 8/9 segments.
A large number of very minute nephridiopores are present. They are scattered all over the body except for the first two segments. These pores are aperture of the integumentary nephridia, through which metabolic wastes of the body are removed.
Dorsal pores of minute apertures of coelomic chambers are present behind the 12th segment which is located mid-dorsally, one in each intersegmental groove, except the last groove. Through these pores, coelom communicates with the exterior.
Earthworms have no eyes, but they have specialized photosensitive cells, called light cells of Hess. These photoreceptor cells have a microvilli-filled central intracellular cavity.
The brains of earthworms are made up of a pair of pear-shaped cerebral ganglia. These are found in the third segment of the dorsal side of the food canal, in a groove between the buccal cavity and the pharynx.
Earthworms have a dual circulatory system in which food, waste, and respiratory gases are carried both by the coelomic fluid and a closed circulatory system.
The excretory system includes a pair of nephridia in every section, except for the first three and the last ones. Integumentary, septal, and pharyngeal are the three forms of nephridia.
Earthworms do not have any separate breathing organs. Gases are exchanged through the wet skin and capillaries, where the haemoglobin dissolved in the blood plasma takes up oxygen and releases carbon dioxide. Water can also be transferred through the skin through active transport, as well as salts.
The musculature (a combined effect of contraction and relaxation of both the muscle layer) of the body wall and seta and the hydrostatic pressure produced by the coelomic fluid is involved in earthworm movements. For forward locomotion, the increase in the hydrostatic pressure of the anterior segments of the body (usually 9 segments) is responsible.
The diagram gives the interior and exterior structure of earthworm.
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How Do Earthworms Reproduce?
Earthworms are hermaphrodites; that is, they have sexual organs, both male and female.
Several common earthworm species are mostly parthenogenetic, meaning that without fertilization, embryo growth and development occurs.
In segments 9 to 15, the sexual organs are found.
Earthworms contain one or two pairs of testicles in sacs.
The sperm is produced, stored and released through the male pores by two or four pairs of seminal vesicles.
In segment 13, the ovaries and oviducts release eggs through female pores in segment 14, while sperm is expelled from segment 15.
External spermatophores are used by certain animals for sperm transmission.
In earthworms, copulation and reproduction are different processes. The front ends ventrally overlap the mating pair and each shares sperm with the other.
In colour, the clitellum becomes very reddish to pinkish. Sometime after copulation, long after the earthworms have separated, the clitellum (behind the spermathecae) secretes material which forms a ring around the worm.
The earthworm then comes back out of the ring, and it injects its eggs and the sperm of the other earthworm into it as it does so.
Each earthworm thus becomes the genetic father of some of its offspring and the rest of its genetic mother.
The ends of the cocoon seal create a vaguely onion-shaped incubator (cocoon) in which the embryonic worms develop, as the worm slips out of the ring. Fertilization is also external.
In the earth, the cocoon is then deposited. Around 2 to 20 young worms hatch after three weeks, with an average of 4.
Direct development without any larva formation.
Types of Earthworms
There are three types of earthworms, and these can be defined by the part of the ecosystem that the worm primarily inhabits.
Let us look into the detailed explanation of types of earthworms.
The Greek translation for 'Epigeic' is 'on the earth' since these worms do not create burrows, and instead live on the soil surface amid a decaying organic matter.
These are also often referred to as compost earthworms, or surface-dwelling earthworms, as they live in piles of leaves or compost heaps on the surface of the soil.
They feed on plant matter that is rotting, leaf litter, and dung.
They are weak burrowers, and between loose organic materials and topsoil, they tend to live.
They have a dark colouring that helps them to live more comfortably above ground, camouflaging themselves in leaf piles or topsoil.
Their dark pigmentation helps to shield them from UV rays as well.
They have powerful muscles for their size, allowing them to run faster than other forms of worms, which is important since they are most at risk from predators living above ground.
In composting, these worms are important and are known to eat and excrete composting material quickly to help it break down more rapidly.
They can also replicate very rapidly, increasing the worm population inside the compost.
They are small in size, typically ranging in length from less than an inch to seven inches.
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In Greek, 'Endogeic' means 'under the earth,' and accordingly, these worms burrow within the top layers of soil and seldom come to the surface, choosing literally to live within the earth instead.
In the uppermost layers of soil, they are most often found where they build semi-permanent, horizontal burrows or under rocks and logs, although some will burrow deep into the soil.
In cases of heavy rain, they usually only make an appearance on the ground surface, as the extra moisture prevents them from drying out.
These worms are fairly tiny and range between one and twelve inches in general.
They appear to be very pale or transparent and colourless, and they have muscles that are weaker than epigeic worms, meaning that they move more slowly.
When they consume the soil itself, they help to balance minerals and air within the soil and help with aeration.
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In Greek, 'Anecic' means 'out of the earth' because while these worms live below ground, they come to the surface of the soil for their food.
Such worms are those that vertically burrow in the soil's mineral layers, forming permanent burrows as deep as six feet below surface level.
Their burrow structures are very comprehensive and can be as wide in diameter as one inch.
In the form of organic matter, such as fallen leaves, these worms gather food from above ground and drag it back underground to their burrows.
It is also known that they consume soil and some debris.
They have very weak muscles and, since they do not need to move fast, are the slowest moving of all forms of worms.
They have some pigmentation but are often a milky colour, as they mainly reside underground, particularly native worms.
They can vary in size dramatically, from one inch to a massive sixty inches in length everywhere.
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While some earthworms prefer mud, such as the mud that is found along the shores of lakes or swamps, the habitat of the earthworm is mostly in moist land.
In the soil of backyards, as well as near bodies of fresh and saltwater, earthworms can be found.
In the topsoil, many earthworms live, while others dwell deeper in the soil.
Earthworms can also be present in the soil found in tree branches in tropical regions.
Benefits of Earthworm
The earthworm information of its benefits are as follows:
Earthworms mainly feed on decomposing organic matter such as leaves and dead plant roots. Such nutrients are concentrated in the digestive system of the worm and released into the excreted earthworms cast back into the soil. These casts are nutrient-rich, especially nitrogen and phosphorus, and are often left in the burrows of worms. The soil's surface under the burrows contain nutrients that are readily accessible to the plant roots.
Earthworms feed on organic matter that decays and rots, and this process helps to break down the materials, allowing bacteria and fungi to feed on them.
Although it is nice for our soil and plants to keep worm populations alive, we should also note that worms are an important source of food for many predators, such as birds. Without worm populations to feed on, certain creatures will quickly decline, such as endangered land snails.
Fun Facts about Earthworms
Earthworms have five aortic arches, which technically acts as a “Heart”. So, earthworms have “Five Hearts”.
Earthworms breathe through their skin as they don’t have specialized respiratory organs.