Ascaris lumbricoides: Digestive, Respiration, Excretory, Nervous System and Locomotion

 

Table of Contents

• Digestive System of Ascaris lumbricoides
• Respiration in Ascaris lumbricoides
• Excretory system of Ascaris lumbricoides
• Nervous system of Ascaris lumbricoides
• Sense organs of Ascaris lumbricoides
• Locomotion of Ascaris lumbricoides
• References

Digestive System of Ascaris lumbricoides

I) Alimentary canal

The alimentary canal is a straight and complete tube that extends from the mouth at the anterior end to the anus at the posterior end. It consists of a short pharynx or esophagus, representing the foregut, a long intestine or midgut, and a short rectum or hindgut.

1. Mouth

• A triradiate aperture.
• Located at the anterior tip.
• Guarded by three lips or labia.

2. Pharynx

• The mouth opens into a short, cylindrical, thick-walled, and muscular pharynx.
• The posterior part of the pharynx has a swelling called the end bulb, which contains valves.
• The pharyngeal wall consists of:
• Syncytial epithelium traversed by radial muscle fibers that dilate the lumen.
• An external bounding membrane.
• An internal cuticle lining that continues with the body wall.
• Contains three branching gland cells, known as pharyngeal or oesophageal glands, which open into the lumen via circular ducts.
• The lumen of the pharynx is triradiate.
• Marginal fibers, composed of connective tissue, are present at the outer end of each ray of the lumen and the outer bounding membrane. These fibers help maintain the triradiate shape of the lumen.

3. Intestine

• The pharynx is followed by a thin-walled, dorsoventrally flattened intestine (midgut).
• Extends almost the entire body length.
• The intestinal wall consists of:
• A single layer of tall columnar cells.
• An external basement membrane and a thin layer of cuticle.
• The inner margins of the intestinal cells have hair-like projections called microvilli, increasing the surface area for absorption.
• The microvilli are formed by a bacillary layer.
• Lacks muscle fibers.

4. Rectum

• The last part of the digestive system.
• The intestine is followed by a short, dorsoventrally flattened rectum.
• The rectal wall consists of:
• A thin layer of tall columnar epithelial cells.
• An internal cuticle lining that continues with the body wall.
• An external muscle tissue layer.
• In males, the rectum opens into the cloaca, which also receives the ejaculatory duct.
• In females, the rectum opens through the anus, which is:
• Guarded by lips.
• Equipped with special dilator muscles (depressor ani) that connect the rectum to the body wall.
• The contraction of these muscles facilitates the discharge of fecal matter.
• The anus or cloaca is located approximately 2 mm from the tail end.
• Contains large unicellular rectal glands:
• Three in females.
• Six in males.

II) Food, feeding, and digestion

• The food of Ascaris consists of blood, tissue exudates, and partially or fully digested food in fluid form from the host’s gut.
• The pharynx generates a suctorial action to suck food.
• Digestion is completely extracellular and takes place in the intestine.
• Enzymes such as proteases, amylase, and lipase, secreted by the pharyngeal gland cells, aid in digestion.
• Digested food is absorbed by the intestinal cells and distributed through pseudo coelomic fluid.
• Excess food is stored mainly as glycogen, with small amounts of fat deposited in the intestinal wall, muscles, and syncytial epidermis.
• Intercellular digestion occurs in intestinal cells, where they engulf small solid particles via phagocytosis and digest them intracellularly.
• Undigested food is expelled by the depressor ani muscle, which raises the dorsal wall of the rectum and the posterior lip of the anus or cloaca.

Respiration in Ascaris lumbricoides

• Ascaris lumbricoides lacks respiratory organs.
• It respires anaerobically (anoxybiotically) due to the low oxygen content in the host’s intestine, similar to other endoparasites.
• During respiration, glycogen is broken down into carbon dioxide, fatty acids, and energy through glycolysis, with waste products excreted through the cuticle, similar to flatworm parasites.
• The main fatty acids produced as excretory wastes include valerianic, butyric, and caproic acids.
• Aerobic respiration may occur when free oxygen is available in the host's intestine, as indicated by the presence of a small amount of cytochrome in the parasite.
• A small quantity of hemoglobin in the body wall and pseudocoelomic fluid helps in oxygen uptake, even when oxygen is scarce.

Excretory system of Ascaris lumbricoides

• The excretory system of Ascaris lumbricoides is simple, lacking flame cells or protonephridia.
• It consists of a single giant H-shaped rennet cell that forms the excretory system.
• The system includes two lateral longitudinal excretory canals connected below the pharynx by a transverse canalicular network.
• Tunnel-like structures in the cytoplasm of rennet cells function as excretory canals.
• The two longitudinal excretory canals (right and left) extend along the body length and are closed at both ends.
• The left canal is more developed than the right.
• The canals are lined by a firm membrane, covered with a layer of cytoplasm, and lack cilia.
• A short terminal excretory duct extends from the left side of the transverse canalicular network to the excretory pore, located near the anterior end.

Physiology:

• The excretory process is not well understood.
• Urea is the main excretory product, diffusing into the pseudocoelomic fluid, making Ascaris ureotelic.
• The excretory canals collect waste from different body parts and eliminate it through the excretory pore.
• Pseudocoelomic fluid pressure aids in ultrafiltration.
• Some ammonia and urea are also expelled through the anus.

Nervous system of Ascaris lumbricoides

• Ascaris lumbricoides has a well-developed and complex nervous system, located in the body wall (hypodermic), similar to the excretory system.
• According to Goldschmidt (1908-1910), the nerve cells in the system are constant in number, position, form, and fiber arrangement.

1. Central nervous system

• Consists of a ganglionated nerve ring (circumenteric ring) around the pharynx.
• Composed of nerve fibers and diffusely arranged nerve cells.
• The nerve ring has multiple ganglia:
• Unpaired dorsal ganglion with a pair of sub-dorsal ganglia nearby.
• A pair of lateral ganglia, further divided into six ganglia.
• A pair of large ventral ganglia on the lower side of the ring.
• Each ganglion contains a fixed number of nerve cells.

2. Peripheral nerves

• Eight nerves extend from the nerve ring to supply the anterior body parts:
• Six nerves supply the six labial papillae, each with a papillary ganglion at its base.
• Two amphidial nerves supply the amphids, originating from amphidial ganglia in the lateral ganglia.
• Six long nerves supply the posterior body parts:
• Mid-dorsal nerve runs along the dorsal line.
• Mid-ventral nerve runs along the ventral line, is ganglionated, and functions as a nerve cord.
• Dorsal nerve cord runs through the dorsal epidermal chord.
• Ventral ganglionated nerve cord runs through the ventral epidermal chord and terminates at the posterior end, forming an anal ganglion.
• Four thinner posterior nerves include:
• One pair of dorsolateral nerves.
• One pair of ventrolateral nerves, located near the excretory canals.
• Several transverse commissures are asymmetrically arranged along the body, connecting the ventral nerve cord with the lateral and dorsal nerve cords.

3. Rectal nervous system

• In males, the posterior end contains:
• Lateral nerve cords, supplying the pre-anal papillae.
• Ventral nerve cord, supplying the post-anal papillae.
• Dorsal, ventral, and lateral nerve cords are interconnected.
• In females, this complex rectal nervous system is absent.

Sense organs of Ascaris lumbricoides

Ascaris lumbricoides has well-developed sense organs, which appear as small elevations or pits in the cuticle.

These sense organs are supplied by nerves and include papillae, amphids, and phasmids.

A. Papillae

Papillae are small villi-like structures located on various parts of the body.

1. Labial Papillae

• Consist of sensory cells surrounded by supporting cells.
• Function as gustatoreceptors (taste organs).
• Located on the three lips surrounding the mouth.
• Four labial papillae are present:
• Two on the dorsal lip.
• One on each ventrolateral lip.
• Each papilla acts as a double sense organ.

2. Cervical papillae

• A pair of small pits located 2 mm behind the lips dorsally.
• Contain bulb-like nerve endings with supporting cells.
• Function as tactile organs.

3. Anal papillae

• Comprise nearly 50 pairs of pre-anal papillae and 5 pairs of post-anal papillae.
• Formed by 1-3 nerve fibers embedded in supporting cells.
• Located ventrally at the posterior end of males.
• Function as tactile organs, assisting in copulation.

B. Amphids 

• Two amphids, located one on each latero-ventral lip.
• Appear as small pits containing glandular and nerve cells.
• Innervated by the amphidial nerve from the lateral or amphidial ganglia.
• Function as chemoreceptors (gustatory sensory organs).

C. Phasmids

• Pit-like chemoreceptors, functioning as unicellular glands.
• Located one on each side of the tail, behind the anus.

Locomotion of Ascaris lumbricoides

• Ascaris lumbricoides has restricted body length changes due to the absence of circular muscles in the body wall.
• The fibers of the cuticle allow only limited length variation.
• While the fibers themselves are inelastic, their spiral and mesh-like arrangement enables some flexibility.
• Locomotion is achieved through undulating movements, facilitated by the alternate contraction of dorsolateral and ventrolateral muscles at the anterior end.
• This movement helps counteract the peristaltic activity of the host’s intestine.

References

• Kotpal, R.L. (2017). Modern Text Book of Zoology: Invertebrates (11th Edition). Rastogi Publications.
• Jordan, E.L., & Verma, P.S. (2018). Invertebrate Zoology (14th Edition). S Chand Publishing.
• Centers for Disease Control and Prevention (CDC). (n.d.). DPDx - Ascariasis.
• Wikipedia Contributors. (2023). Ascaris lumbricoides. In Wikipedia, The Free Encyclopedia.
• This article provides an overview of the species, including its morphology, life cycle, and impact on human health.
• Wikipedia Contributors. (2023). Ascaris. In Wikipedia, The Free Encyclopedia.
• This entry discusses the genus Ascaris, highlighting the characteristics and significance of species like Ascaris lumbricoides.