Hint: The organ systems of multi-cellular systems evolved which divided the metabolic needs of the body. Individual organs evolved to perform the excretory function. Excretory cells called flame cells developed in flatworms, while nephridia developed in annelids.
Complete step by step answer: A. Earthworms, arthropods, and mollusks have evolved excretory structures called nephridia. A pair of nephridia is present on each segment of the earthworm. They're the same as flame cells therein they need tubules with cilia and performance sort of a kidney to get rid of wastes, but they often receptive the outside of the organism. The ciliated tubules filter fluid and carry waste through openings called nephrostomes. From the nephrostomes, excretion occurs through a pore called the nephridiopore. A nephridium is more evolved than a urinary organ in this it's a system for reabsorption of some useful waste products, like metabolites and ions, by a capillary network before excretion.
B. These are of three types:
1. Septal Nephridia:
These are found on the intersegmental septum between the 15th and 16th segments to the posterior side of the body.
Each septum bears nephridia on both the surfaces arranged in semicircles round the intestine, two rows ahead of the septum, and two behind it. Each segment possesses 80 to 100 septal nephridia except the 15th segment which has only 40 to 50 nephridia. These don't seem to be found within the segments up to 14th.
The septal nephridia are also considered typical of all the nephridia of Pheretima. Each septal nephridium consists of nephrostome, neck, the body of nephridium, and therefore the terminal duct.
2. Integumentary Nephridia:
In each segment, from the 7th to the last segment, numerous nephridia are found attached inside the liner of the body wall. These are called integumentary nephridia.
These are small-sized, lacking the nephrostome, and with none opening into the coelom.
Hence, they're called a closed kind of nephridia.
These discharge excretory wastes directly outside, hence, they're called exonephric nephridia.
3. Pharyngeal Nephridia:
These nephridia lie three paired tufts, one on either side of the anterior region of the epithelial duct within the segments 4th, 5th, and 6th. The tufts of pharyngeal nephridia also contain blood glands.
Each pharyngeal nephridium is about the dimensions of a septal. It's a brief straight lobe and a spirally twisted loop, its lumen has ciliated canals. Ductules arise from each nephridium and unite to create one thick-walled duct on either side in each segment.
The two ducts of nephridia of segment 6th open into the bodily cavity in segment 2nd and therefore the paired ducts of nephridia of segments 4th and 5th open into the pharynx in segment 4th.
These nephridia discharge their wastes into an epithelial duct and are, therefore, enteronephric but such enteronephric nephridia which open into the anterior region of the alimentary tract are called protonephridia because they will have taken the function of digestive glands.
Note:Nephridia are more evolved than flame cells as they have the ability to reabsorb useful metabolites before excretion of waste.
Both nephridia and flame cells are ciliated tubules that filter fluids within the cell to get rid of waste.
Flame cells are connected to a duct system to expel wastes, while nephridia are present outside of the organism.