Excretory System in Animals As living creatures, animals also have an excretory system.There are various excretory systems in animals. Below we will discuss the animal excretory systems one by one. 1. Platyhelminthes The excretory organs of Platyhelminthes, like those of Planaria, are vibrating hair cells. Because these vibrating hairs look like flames, these cells are called flame cells. Body fluids are filtered in the flame cell and waste substances are absorbed and then excreted from the body through holes on the surface of the body. Excretory organs in Planaria 2. Annelids Annelids already have special excretory organs, namely in the form of nephridia which are found in each body segment. each segment there is a pair of nephridia. These nephridia are equipped with open, ciliated funnels called nephrostomes which are found on each segment separating the segments. The nephrostom functions to attract and take in body fluids. When fluid passes through the nephridia, useful substances are absorbed by the blood and waste substances, such as water, nitrogen compounds, and salts that are not needed by the body are stored in the urinary bladder. The waste substances are then excreted through the nephridiophore (nephridium hole). An example of an annelid that we can easily find is earthworms. Earthworms excrete urine per day amounting to 60% of their body weight. Structure of nephridia in earthworms 3. Insecta In insects such as cockroaches and grasshoppers, the excretory organs are smooth yellowish tubes called Malpighian vessels. Malpighian vessels function to remove urea, uric acid and salts from the blood to the intestines. The number of Malpighian vessels varies. This vessel is connected to the intestinal tract at the border of the middle intestine and the hindgut.Metabolic waste substances are absorbed from tissue fluid by Malpighian vessels and form uric acid crystals. This uric acid enters the hind intestine and eventually comes out with the feces.Some of the waste substances containing nitrogen are used to form chitin in the exoskeleton and can be excreted during molting. 4. Pisces The fish's excretory organs are a pair of opistonefros kidneys which are the most primitive type of kidney. In opistonefros kidneys, the anterior tubules have disappeared, some of the middle tubules are connected to the testes, and there is a concentration and multiplication of tubules in the posterior.The excretory mechanism of fish that live in fresh water is different from that of fish that live in sea water. Fish that live in fresh water excrete ammonia and actively absorb inorganic ions through their gills and excrete large amounts of urine.On the other hand, fish that live in the sea excrete nitrogenous waste in the form of trimethylamine oxide (TMO) which gives marine fish a distinctive odor, produce ions through the gills, and excrete a small amount of urine. Saltwater fish kidneys do not have glomeruli. As a result, ultrafiltration does not occur in the kidneys and urine formation is entirely due to the secretion of salts and TMO related to water osmosis. Excretion mechanism of freshwater fish Excretion mechanism of freshwater fish