The Earthworm is a reddish-brown terrestrial invertebrate that dwells in the upper layer of moist soil It belongs to phylum Annelida. It feeds on a wide variety of organic matter and lives in burrows made by swallowing and boring the soil. Pheretima posthuma and Lumbricus terrestris are the most common Indian earthworms. The earthworm’s faecal deposits are known as worm castings and can be located in gardens and moist soil.
The biological process of elimination of toxic and the waste products, such as nitrogenous material including ammonia, urea, amino acid, uric acid etc, from the body is known as excretion.
Excretory system of earthworms is Nephridia. A pair of Nephridia is present in all segments of the body of earthworm except for the first three segments and last segment.
Types of Nephridia
Based on structure and location there are three types of Nephridia.
- Septal Nephridia or Enteronephric Nephridia.
- Integumentary Nephridia or Exonephric Nephridia.
- Pharyngeal Nephridia.
Septal Nephridia or Typical Nephridia
Septal nephridia are well-developed and the largest Nephridia present in the intersegmental septum. Septal nephridia start from the 15th or 16th segment and continue until the second last segment. They are located on both sides of septum in each segment. About 80 to 100 septal nephridia are present on each side of the septum.
A septal nephridia consists of : nephrostome (Ciliated funnel), neck, body and terminal duct. The body of the nephridium consists of a long spirally twisted loop with a narrow apical part, a short straight lobe and a terminal duct, which joins the septal excretory canal. Only septal nephridia has a funnel. This funnel-shaped nephrostome is externally ciliated and is round in structure. It opens into the coelomic activity. The main function of Septal nephridia is to discharge the waste products through the canal and ducts into the lumen of the intestine and hence they are enteronephric.
Integumentary nephridia: Also called the micro nephridia, Integumentary nephridia are the smallest nephridia of Pheretima. They lie dispersed on the whole internal surface of the body wall except in the first two segments. They hang out in the coelom and are V-shaped structures. The number ranges from 200-250 in each segment, except in the clitellar segments in which they are present ten times more than the usual number.
This large number of the integumentary network located in the clitellar region is known as forests of nephridia. The main function of this excretory system of Earthworm is to discharge nitrogenous waste products directly outside from the body through nephridiopores and hence are called mesonephric nephridia.
Pharyngeal nephridia: Located in 4th, 5th and 6th segments, which lie on either side of the oesophagus, pharynx and paired bunches. Hence, they are also called tufted nephridia. Each tufted nephridia consists of a large number of nephridia. The terminal ducts of these nephridia join together and form a long, thick-walled duct, which opens into the gut.
The vents from the nephridia of the fourth and fifth segments open into the pharynx, whereas those from the nephridial bunches of the sixth segment open into the buccal cavity. These nephridia are closed types since they lack the nephrostome. Therefore, the waste products are directly discharged into the buccal cavity and pharynx. These are taken outside with undigested food through the anus called enteronephric nephridia.
Uric acid is not found in earthworms. Amino acids are further broken down to form free ammonia and urea.
Excretory System of Cockroaches
Cockroaches belong to the class Insecta of Phylum Arthropoda. Their bodies are divided into head, thorax and abdomen. Normally, they are nocturnal omnivores living in damp and dark places devoid of light.
The excretion organ of the cockroach is the Malpighian tubules. Each tube is fringed by glandular and ciliated cells that absorb and soak up nitrogenous waste products. This is then converted into uric acid, which is eliminated through the hindgut. After reabsorption, the waste moves to the rectum and is released through the anus. This insect is uricotelic. In addition to malpighian tubules, nephrocytes present in the lateral wall of the heart and Uricose glands( present in males of some species) also help in excretion and store nitrogenous waste.
Excretory System of Frogs
Frogs belong to class Amphibia of phylum Chordata. They can survive both on land and in freshwater. Their body temperature varies depending on the environment’s temperature and is not constant. Such animals are called poikilotherms or cold-blooded animals.
A highly advanced excretory system carries out the elimination and excretion of nitrogenous wastes. Excretory system contains ureters, cloaca, kidneys and the urinary bladder. Kidneys are dark red, compact, and bean-like structures located posteriorly into the human body cavity on either side of the vertebral column. Kidneys contain several functional and structural units known as uriniferous tubules of nephrons.
Each nephron consists of 2 main parts – a uriniferous tubule and a Malpighian body. The malpighian body consists of the Bowman’s capsule, which is a double-layered, thin-walled, cup-like structure and a network of blood capillaries called the glomerulus. The Uriniferous tubule forms Bowman’s lower portion, opening into the collecting tubule. All collecting tubules combine to extend into the ureter. The malpighian body initiates the process of filtration. The filtrate passes down into the body of the uriniferous tubule, where the body reabsorbs much water, glucose, and other useful blood salts. Hence frogs are ureotelic.
In the male frogs, two ureters evolve out of the outer margin of kidneys. The ureters open into the cloaca and act as a urinogenital duct. The oviduct and ureters in females open separately into the cloaca.
Conclusion
The excretory system of earthworms (annelids) has slightly more evolved excretory structures called nephridia. A pair of nephridia is present on each segment of the Earthworm. They are similar to flame cells in that they have a tubule with cilia. In Cockroaches, malpighian tubules perform excretion.
This waste, once converted into uric acid, is excreted through the hindgut. Therefore, the cockroach is known as Uricotelic. Additionally, the body fat bodies, uricose and nephrocytes gland helps in the excretion. The excretory system of the frog consists of kidneys: Urinary bladder, cloaca, and ureter. The excretory wastes by blood, excreted and separated into the kidney. The frog excretes urea.