Malpighian Tubules
Other arthropods like insects and myriapods and arachnids have Malpighian tubules, the outgrowths of alimentary canal like excretory organs. Malpighian tubules are totally new structures, containing no resemblance either to nephridia or coelomoducts. Generally, the Malpighian tubules that are composed of single layered epithelium and are bathed in the blood of haemocoel. By the procedure of active secretion water passes into the lumen of Malpighian tubule together with nitrogenous wastes and dissolved salts. The secretion essentially takes place in the distal parts of the tubules. The proximal part of the tubule as well as the rectum is the sites of absorption. Most of the terrestrial arthropods excrete uric acid. The reabsorption of water results in the precipitation of urate crystals as nitrogenous waste, therefore the animals are termed as uricotelic. Peripatus which is also terrestrial lacks Malpighian tubules. Nevertheless it is uricotelic. The excretion of uric acid is acquired from the intestinal epithelium into the intestinal lumen. It is removed from the intestine within a peritrophic membrane which is sloughed off every 24 hours. There are also segmentally arranged coelomic excretory organs. The coelomoducts of Peripatus can be considered as vestigial organs of its aquatic ancestry, with a much reduced activity.