Excretion in Hydra:
In hydra nitrogenous waste is in the form of ammonia. Almost all the cells of Hydra are in direct contact with water, so ammonia is removed by simple diffusion from the external surface as well as internal surface into external and internal water of gastrovascular cavity.
Excretion in Plavaria:
Free living flatworms like plavaria have developed a tubular excretory system. In Plavaria the excretory system consists of two longitudinal branching tubules or excretory canals lying on either lateral sides and extending along the entire length of the animal. Each tubule opens to the external surface of the animal by many excretory pores. Internally in the mesenchyme, each tubule gives rise to numerous blind bulb like cells called flame cells or pronephridia which are bathed in the tissue fluids. Each flame cell is hollow inside and bears a tuft of cilia which beat in manner like flickering flame (hence called flame cell), water along with ammonia diffuse from the tissue fluid into the lumen of flame cells. The beating of cilia propels this solution into excretory canal where it is to be excreted out by excretory pores.
During the movement of excretory fluid, water is being reabsorbed if required by the animal and the rest of the excretory fluid (urine) is passed out in the form of hypotonic solution. Thus it seems that flame cell functions mainly in osmoregulation and most metabolic wastes are removed from body surface or excreted in the gut where they are removed through the mouth along with undigested food. In some parasitic flatworms which are isotonic to the body fluids of their hosts, the flame cell, perform excretion of nitrogenous wastes.
Excretion in earthworms:
Earthworms have combined excretory and osmoregulatory organs called metanephridia, which are arranged segmentally. Each metanephridium is highly coiled tubule immersed in coelomic fluid and surrounded by a network of capillaries. It is opened at both the ends. Its internal opening called nephrostome, which lies in the coelom is a ciliated funnel like structure. While the external minute opening or nephridiopore opens outside in the skin. The coiled tubular part dilates finally to form a bladder before opening to outside through nephridopore. Due to beating of cilia of nephrostome, coelomic fluid is pumped into excretory tubule. Some excretory substances are also secreted by cells of tubule. Here selective re-absorption of useful substances also occurs which are taken back by the blood into the circulation. Finally the excretory fluid (urine) is emptied in the bladder which excretes it outside through nephridiopore.
Excretion in Cockroach:
Cockroach and other insects have excretory system which consists of long, thin blind tubules called malpiglian tubules which arise from the junction of midgut and hindgut. Malpiglian tubules are immersed in the haemolymph (fluid of body cavity), cells of malpiglian tubules absorb excretory wastes along with some useful substances present in haemolymph. In the latter part of the tubule selective re-absorption of useful substances occurs and the uric acid is discharged into the rectum. Rectum stores uric acid for re-absorption of salts and water, so the uric acid becomes almost dry and pass out along with faeces.
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