Ascaris lumbricoides- Habitat and External Morphology

Ascaris lumbricoides is a parasitic worm that belongs to the phylum Nematoda, the class Chromadorea, the order Ascaridida, and the family Ascarididae . It is the most common and largest intestinal nematode that infects humans, especially children . It can grow up to 40 cm in length and 6 mm in diameter . It has a cylindrical, elongated, and tapered body with a pseudocoelom, a mouth with three lips, and a cuticle with transverse striations . It shows sexual dimorphism, meaning that males and females have different features. Males are smaller than females and have a curved tail with copulatory spicules, while females have a straight tail and a vulva near the anterior end .

Ascaris lumbricoides causes ascariasis, a disease characterized by abdominal discomfort, malnutrition, growth retardation, intestinal obstruction, and sometimes respiratory complications . The infection occurs through the fecal-oral route, when humans ingest eggs that are shed in the feces of infected individuals and contaminate soil, water, or food . The eggs hatch in the small intestine and release larvae that migrate through the blood vessels to the lungs, where they break into the alveoli and ascend to the trachea. The larvae are then swallowed again and reach the small intestine, where they mature into adult worms and reproduce . The life cycle takes about two to three months to complete.

Ascaris lumbricoides is widely distributed in tropical and subtropical regions of the world, where sanitation and hygiene are poor. It is estimated that 807 million to 1.2 billion people are infected with this worm worldwide. It can also infect pigs and other animals, and some studies suggest that Ascaris lumbricoides and Ascaris suum (pig roundworm) are the same species. Ascariasis can be treated with anthelmintic drugs that kill or expel the worms from the body. Prevention measures include improving sanitation, washing hands and food before eating, boiling or filtering water, and deworming programs for children.

Ascaris lumbricoides is a parasitic nematode that lives in the small intestine of humans and some other animals. It is one of the most common intestinal worms in the world, infecting about 1.5 billion people, especially in tropical and subtropical regions.

The life cycle of Ascaris lumbricoides involves two hosts: a definitive host (human or animal) and an intermediate host (soil). The adult worms mate and produce eggs in the small intestine of the definitive host. The eggs are passed out with the feces and contaminate the soil. The eggs can survive for months or years in moist and warm soil conditions. Under favorable conditions, the eggs develop into infective larvae within 2 to 4 weeks. The larvae can penetrate the skin of humans or animals that come into contact with the soil, or they can be ingested with contaminated food or water. The larvae then migrate through the bloodstream and lungs to reach the throat, where they are swallowed and return to the small intestine to mature into adult worms. The whole cycle takes about 2 to 3 months.

Ascaris lumbricoides can also cause aberrant infections, where the worms migrate to other organs or tissues than the small intestine, such as the liver, pancreas, appendix, bile ducts, or even the eyes or brain. This can happen when there is a high worm burden, a heavy reinfection, or an immune response that triggers the worms to move. Aberrant infections can cause serious complications such as obstruction, inflammation, abscesses, perforation, or bleeding.

Ascaris lumbricoides is a roundworm that belongs to the phylum Nematoda. It has a simple and symmetrical body plan that is adapted to its parasitic lifestyle. The external morphology of Ascaris lumbricoides can be described as follows:

• The body is elongated, cylindrical, and gradually tapering at both ends. The anterior end is more slender than the posterior end. The body surface is smooth and glossy, covered by a thick cuticle that protects the worm from the host`s digestive enzymes.
• The body shows sexual dimorphism, meaning that males and females have different features. The females are larger and longer than the males, measuring 20 to 40 cm in length and 4 to 6 mm in diameter. The females have a straight tail and a mid-ventral genital pore near the anterior third of the body. The males are smaller and shorter than the females, measuring 15 to 31 cm in length and 2 to 4 mm in diameter. The males have a curved tail with a pair of copulatory spicules that protrude from the cloaca, which is a common opening for the digestive and reproductive systems.
• The body has four longitudinal lines that run along its length: one mid-dorsal, one mid-ventral, and two lateral. These lines are visible externally and are formed by the projections of the underlying epidermis. The dorsal and ventral lines are white, while the lateral lines are brown. The lateral lines also contain the excretory canals and the lateral nerves of the worm.
• The anterior end has a small terminal mouth that is surrounded by three broad lips or labia. One lip is mid-dorsal and elliptical, while the other two are latero-ventral and oval. Each lip bears sensory papillae that help the worm to detect touch and vibrations. The dorsal lip has two papillae, one on each side, while each latero-ventral lip has one double papilla on its ventral side. These four papillae form an outer labial circle. The latero-ventral lips also have lateral papillae that open into cuticular cavities called amphids, which are chemoreceptors that help the worm to sense chemicals. Behind the latero-ventral lips, there is a pair of cervical papillae that are close to the nerve ring, which is a circular concentration of nerve cells around the pharynx.
• The posterior end also shows sexual dimorphism. In females, the posterior end is conical and straight, with a transverse anus that is guarded by a pair of postanal papillae. Only the digestive system opens to the outside through the anus. In males, the posterior end is curved ventrally in a hook shape, with a cloaca that opens to the outside through a pair of preanal papillae. Both the digestive and reproductive systems open into the cloaca. The male tail also has numerous genital papillae that help in copulation. Two anterior pairs of postanal papillae are double, while the rest are single.

The external morphology of Ascaris lumbricoides reflects its adaptation to its parasitic mode of life. It has a streamlined body that can easily move through the host`s intestine, a protective cuticle that resists digestion, sensory structures that help it to find food and mates, and reproductive organs that ensure its propagation.

Ascaris lumbricoides is characterized by its great size and cylindrical shape. It gradually tapers at both ends, with the anterior end being more slender than the posterior end . It shows sexual dimorphism, meaning that the sexes are separate and have different features . The female worm is larger than the male worm, measuring 20 to 49 cm (7.9 to 19 in) in length and 3 to 6 mm (0.1 to 0.2 in) in diameter . The male worm is smaller, measuring 15 to 31 cm (5.9 to 12 in) in length and 2 to 4 mm (0.08 to 0.2 in) in diameter . The male`s posterior end is curved ventrally and has a bluntly pointed tail, while the female`s posterior end is conical and straight .

The following table summarizes the shape and size of Ascaris lumbricoides:

The large size of Ascaris lumbricoides makes it visible to the naked eye and easily identifiable in stool samples or expelled worms. However, the size may also cause problems for the host, such as intestinal obstruction, perforation, or migration of the worms to other organs. Therefore, it is important to diagnose and treat Ascaris lumbricoides infection promptly and effectively.

The color of Ascaris lumbricoides varies depending on its stage of life and location. When freshly passed from the intestine, Ascaris is light brown or pink in color, but gradually changes to white. This is because the cuticle of the worm is stained by bile and other substances in the intestinal lumen. The cuticle also reflects light differently depending on its thickness and structure.

The fertilized eggs of Ascaris lumbricoides are broadly oval, with a thick, mammillated coat, usually bile stained a golden brown. The mammillated coat is a layer of protein and lipid that protects the egg from harsh environmental conditions. The unfertilized eggs are smaller, thinner, and more elongated than the fertilized ones, and have a smooth coat that is not bile stained.

The larvae of Ascaris lumbricoides are transparent and colorless when they hatch from the eggs in the small intestine. They then migrate through the bloodstream to the lungs, where they molt twice and become larger and more pigmented. The larvae are whitish or yellowish in color when they reach the throat and are coughed up or swallowed. They then return to the small intestine, where they mature into adult worms.

The adult worms of Ascaris lumbricoides are cylindrical, with a tapering anterior end and a blunt posterior end. The females are larger than the males, measuring 20 to 40 cm in length and 4 to 6 mm in diameter, while the males measure 15 to 31 cm in length and 2 to 4 mm in diameter. The males also have a ventrally curved tail with two copulatory spicules. The adult worms are typically pink or white with tapered ends. They have four longitudinal lines on their body surface: one mid-dorsal, one mid-ventral, and two lateral. The lines are visible externally and are thickened impressions of the syncytial epidermis. The dorsal and ventral lines appear pure white, while the lateral lines appear brown.

• The cylindrical body of Ascaris lumbricoides has four longitudinal lines that run along its entire length .
• These lines are visible externally and are thickened impressions of the syncytial epidermis .
• The lines are one narrow mid-dorsal, one mid-ventral, and two thick ones are lateral .
• The dorsal and ventral lines appear pure white, while the lateral lines appear brown .
• The excretory pore is situated mid-ventrally, a little behind the lips .
• The genital pore and anus open separately in the female; the genital pore is situated mid-ventrally at about 1/3rd from the anterior end .

The anterior end of Ascaris lumbricoides is the front part of the worm that contains the mouth and some sensory structures. The anterior end is rounded and has a small terminal mouth opening that is surrounded by three broad lips or labia . The lips are used for feeding and attachment to the intestinal wall. They also bear sensory papillae that help the worm detect chemical and mechanical stimuli .

The mouth leads to a muscular pharynx or esophagus that pumps food into the intestine . The pharynx is divided into three parts: an anterior corpus, a middle isthmus, and a posterior bulb. The bulb contains a valve that regulates the flow of food into the intestine.

The anterior end of Ascaris lumbricoides also contains some nervous structures that coordinate the worm`s movements and responses. The most prominent of these is the nerve ring, which is a circular bundle of nerve fibers that surrounds the pharynx . The nerve ring connects to four longitudinal nerve cords that run along the body: one dorsal, one ventral, and two lateral . The nerve cords innervate the muscles, glands, and sensory organs of the worm .

The anterior end of Ascaris lumbricoides is important for its survival and reproduction. It helps the worm feed on the host`s intestinal contents, attach to the host`s mucosa, sense its environment, and mate with other worms .

The posterior end of the body shows clear sexual dimorphism. In females, the posterior extremity is conical and straight. The midventral transverse aperture, or anus, is guarded by one pair of postanal papillae. Only the digestive tubes open to the outside through the anus.

In males, the tail end is curved ventrally in the form of a hook with a conical tip. In males, the anus is replaced by a cloaca. It is the common aperture for the rectum and genital tubes. Two copulatory setae protrude from the aperture. They are called peneal setae or spicules. The setae serve to transfer sperm into the female vagina during copulation. The male tail is characterized by numerous genital papillae ventrally, about 50 pairs of pre-anal papillae, and 5 pairs of postanal papillae. Genital papillae of male help in copulation. Two anterior pairs of postanal papillae are double, while the rest are single.

The excretory system of Ascaris lumbricoides is simple and consists of a single giant H-shaped rennet cell that forms an excretory canal. The canal has two lateral longitudinal branches that run along the lateral chords of the body and are connected by a transverse network below the pharynx . The excretory canal collects the excretory products from different parts of the body and eliminates them through the excretory pore .

The excretory pore lies mid-ventrally and near the anterior end, just behind the lips . It is a small opening that is lined by the cuticle and surrounded by a few cells. The excretory pore is the only outlet for the waste products of Ascaris lumbricoides, as it has no anus or cloaca.

The main excretory product of Ascaris lumbricoides is urea, which diffuses into the pseudocoelomic fluid from the metabolic activities of the cells . Ascaris lumbricoides is therefore a ureotelic animal, meaning that it excretes urea as its primary nitrogenous waste . Urea is a soluble and less toxic compound than ammonia, which is advantageous for an animal that lives in a confined space with limited water availability.

The excretory system of Ascaris lumbricoides plays an important role in osmoregulation, as it helps to maintain the osmotic balance between the internal and external environment. The excretory system also helps to remove toxic substances and metabolic wastes from the body, thus preventing their accumulation and harmful effects. The excretory system of Ascaris lumbricoides is therefore essential for its survival and adaptation to its parasitic lifestyle.

• The female gonopore or vulva is the opening of the reproductive system in female Ascaris lumbricoides.
• It is located on the ventral surface of the body, about one-third of the way from the anterior end, in a region called the vulvar waist.
• The vulvar waist is a narrow constriction of the body that separates the anterior and posterior regions of the female worm.
• The female gonopore is a small transverse slit that can be seen with a magnifying glass or a microscope.
• The female gonopore leads to a short vagina that connects to two uteri, each containing thousands of eggs.
• The eggs are fertilized by the sperm transferred from the male during copulation and are expelled through the female gonopore into the host`s intestine.
• The female gonopore is also the site of infection by male Ascaris lumbricoides, which can penetrate the vulva and enter the vagina, causing inflammation and damage to the reproductive organs.
• The female gonopore is guarded by a pair of vulval lips that can close to prevent the entry of foreign substances or parasites.
• The vulval lips also have sensory papillae that can detect chemical and mechanical stimuli from the environment or the male worm.

The body wall of Ascaris lumbricoides is composed of three layers: the outer cuticle, the middle epidermis or hypodermis, and the inner longitudinal muscle layer. These layers protect the worm from the host`s digestive juices and provide support and movement.

The cuticle layer is the outermost layer of the body wall of Ascaris lumbricoides. It is a non-cellular, proteinaceous layer that covers the entire surface of the worm and protects it from the host`s digestive enzymes and immune system. The cuticle layer also provides structural support and elasticity to the worm`s body .

The cuticle layer is composed of several sublayers that can be distinguished by light and electron microscopy. The sublayers are :

• Lipid layer: This is a very thin (less than 1000 Å) layer of lipids that coats the outer surface of the cuticle. It acts as a barrier to water loss and penetration of foreign substances.
• Cortical layer: This is a thick (about 5 µm) layer of keratinized protein that forms the main bulk of the cuticle. It consists of an outer cortical layer and an inner cortical layer. The outer cortical layer has radial striations that connect with aster-like processes from the underlying matrix layer. The inner cortical layer has discontinuous strips or rings around the body .
• Matrix layer: This is a spongy (about 10 µm) layer of protein that contains a high amount of sulfur-rich matricin. It consists of an outer fibrillar layer, a homogeneous layer, and a boundary layer. The outer fibrillar layer has fine fibers that run parallel to the long axis of the worm. The homogeneous layer has radial striations that extend from the boundary layer to the fiber layer. The boundary layer resembles the inner cortical layer .
• Fiber layer: This is a thin (about 1 µm) layer of collagen fibers that cross each other at various angles and form three strata. The fiber layer provides strength and flexibility to the cuticle .
• Basement membrane: This is a thin (about 0.5 µm) layer that forms the inner limit of the cuticle. It is composed of glycoproteins and attaches the cuticle to the underlying epidermis .

The cuticle layer is secreted by the epidermis and undergoes periodic molting to allow growth and development of the worm. The cuticle also lines the buccal cavity, esophagus, rectum, cloaca, vagina, and excretory pore. The cuticle plays an essential role in Ascaris survival and adaptation to its parasitic lifestyle.

The epidermis or hypodermis is a thin syncytial layer under the basal lamina of the cuticle. It contains many nuclei but no cell walls. The nuclei lie in four longitudinal epidermal cords that run along the mid-dorsal, mid-ventral, and lateral lines. The lateral cords are more conspicuous and seen on the surface as yellow lines. The excretory canals and lateral nerves run along the lateral cords, while the dorsal and ventral cords contain the dorsal and ventral nerve cords, respectively.

The epidermis of free-living nematodes contains unicellular epidermal glands, but these are absent in Ascaris. The epidermis stores fat and glycogen abundantly. The epidermis also secretes the cuticle and regulates the osmotic balance of the worm.

The epidermis or hypodermis layer of Ascaris lumbricoides is important for its protection, nutrition, and excretion. It forms a continuous layer with the pharynx and rectum`s cuticular lining. It also projects along the lips and papillae of the anterior end and the genital papillae of the posterior end.

The body wall of Ascaris lumbricoides has a single layer of longitudinal muscle cells that are attached to the hypodermis. Unlike some other nematodes, Ascaris does not have any circular muscles in its body wall. The longitudinal muscles are arranged in four columns: two dorsolateral and two ventrolateral. Each column contains about 150 muscle cells that are spindle-shaped and have two parts: a muscular part and a protoplasmic part.

The muscular part is the contractile portion of the cell that lies towards the epidermis. It contains longitudinal fibers that are connected to the cuticle by fine fibers. The muscular part is responsible for the movement of the worm by contracting and relaxing in coordination with the other muscle cells.

The protoplasmic part is a non-contractile portion of the cell that lies towards the body cavity. It is a club-shaped or bladder-like mass of protoplasm that contains a nucleus and a network of supporting fibrils. The protoplasmic part extends into a fibrous process or muscle tail that forms a synapse with the nerve cord. The muscle tails are cellular extensions that transmit nerve impulses from the dorsal or ventral nerve cord to the muscle cells.

The muscle cells of the two dorsolateral columns are connected to the dorsal nerve cord, while those of the two ventrolateral columns are connected to the ventral nerve cord. The dorsal and ventral nerve cords run along the mid-dorsal and mid-ventral lines of the body, respectively. They coordinate the movement of the worm by sending signals to the muscle cells.

The number and arrangement of muscle cells in nematodes are important for identifying different species. Ascaris lumbricoides has more muscle cells than some other nematodes, such as Caenorhabditis elegans, which has only 95 muscle cells in its body wall. The longitudinal muscles of Ascaris lumbricoides enable it to move in a whip-like or sinusoidal fashion through the intestinal lumen of its host.

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