How Many Types of Nephridia are Found in Earthworms

Introduction

Nephridia (singular-nephridium) is a segmentally arranged exterior structure found in many vertebrates. It is a tubule open towards the exterior part of an organism which acts as an organ of excretion or osmoregulation (it is the state where the organism maintains a constant osmotic pressure in the fluids of its body by the control of water and salt concentrations). A nephridium has either ciliated or flagellated cells along with absorptive walls. An earthworm has three types of nephridia based on its location. They are the Septal, Integumentary, and Pharyngeal.

Types of nephridia

Nephridia can be of two types -

Metanephridia - (meta = “after”) this kind of nephridia is a type of excretory gland found in many types of invertebrates such as annelids, arthropods and Mollusca. Metanephridia usually consist of a ciliated funnel which opens in the body cavity or coelom and is connected to a duct that can be variously glandulrarized, folded or expanded (vesiculated) and it typically opens to the organism’s exterior. These ciliated tubules of metanephridia pump water carrying surplus ions, metabolic waste, toxins from food, and useless hormones out of the organism by guiding them down funnel-shaped bodies known as nephrostomers. The waste from the organism is passed out at the nephridiopore. The primary urine produced by the filtration of blood (or a similarly functioning fluid) by an organism is modified into secondary urine through selective reabsorption by the cells which line the metanephridia.

Protonephridia - (proto = “first”) this kind of nephridia is found in the phyla Platyhelminthes, Nemertea, Rotifera and Cordata (lancelets). Protonephridia have the same structure as metanephridia but the internal ciliated funnel is blocked by terminal cells; either flame cells (if it is ciliated) or solenocytes (if it is flagellated). Thus the tubules of protonephridia lack internal openings while maintaining their opening to the organism’s exterior.

In protonephridia, each terminal cell has one or more cilia and their beating inside the protonephridia tube creates an outward direction motion thus partial pressurization in the blind of the tube. This pressurization guides waste fluids from the inside of the animal, all of which are pulled through small apertures in the terminal cells and into the protonephridium. The apertures in the terminal cells are large enough for small molecules to pass, but larger proteins can not pass through it thus retaining them inside the animal. From the end part of the protonephridium, the solutes are guided through the tube, formed by the canal cells, and they exit t the animal from a small opening formed by the nephridiopore.

Types of nephridia in an earthworm

Septal nephridia - the septal nephridia are the largest nephridia present in an earthworm and they are located on both sides of each intersegmental septum after the 15th segment and they open into the intestine. There are almost 40 to 50 septal nephridia present in each septum and they are organised in two rows. Thus, each septum gets 80 to 100 septal nephridia. The septal nephridia consist of 3 main parts -

Nephridiostome - it is also known as nephrostome. This structure is funnel-shaped and cilia are present in the external part, opening into the coelomic cavity. The nephridiostome has a mouth-like opening surrounded by a larger upper lip and a smaller lower lip. Due to the continuous movement of cilia, all the nitrogenous waste of the coelomic fluid is soaked up in the nephridium.

Body - this structure represents the main part and remains coiled around its axis.

Terminal duct - this structure is the end part of the main body. All the terminal duct of a segment opens up into the septal excretory duct. This septal excretory duct collects all the excretory products from the body and transfers them to a pair of the supra-intestinal excretory duct.

Integumentary nephridia - This kind of bacteria occurs in all the segments except the first two segments and they vary from 200 to 250 per segment. Cittellar segments have around 2000 to 2500 each. These structures are attached to the body wall and are closed internally without the nephrostome, and open on the body surface. They are the smallest in structure and remove metabolic wastes from the blood only.

Pharyngeal nephridia - As the name says pharyngeal, it means that these nephridia are found in the pharynx of an earthworm (pharynx is a thick-walled, pear-shaped chamber, that extends till the 4th segment. It is differentiated from the buccal chamber by a dorsal groove in which the brain of an earthworm is located). The pharyngeal nephridia are sound in the 4th, 5th and 6th segments of the earthworm. These nephridia filter nitrogenous waste from the capillaries and then their ends are dumped inside the pharynx and the buccal cavity.