The simplest tubular excretory system is the flame-cell system of flatworms (Phylum Platyhelminthes). These animals have neither circulatory systems nor coeloms, and so the flame-cell system must regulate the contents of the interstitial fluid directly.
The apparatus consists of a branched system of tubules ramifying throughout the body. Each of the smallest tubules at the tips of this excretory tree is capped by a bulbous cell called a flame cell. Interstitial fluid bathing the tissues of the animal passes through the flame cell and enters the tubular system.
The flame cell has a tuft of cilia projecting into the tubule, and the beating of these cilia propels fluid along the tubule, away from the blind end where the flame cell is located. (The beating cilia look like a flickering flame, for which these cells are named.) In planaria, tributaries of the tubular system drain into excretory ducts that empty to the external environment through numerous openings called nephridiopores.
The excreted fluid is very dilute in the case of freshwater flatworms; this helps to balance the osmotic uptake of water from the hypoosmotic environment. The cellular mechanisms for this osmoregulation are unknown, however. It is likely that the lining of the tubules is a transport epithelium specialized for reabsorbing certain salts before the fluid exits from the body.
The flame-cell systems of freshwater flatworms function mainly in osmoregulation; most metabolic wastes are excreted into the gastrovascular cavity and eliminated through the mouth. However, some parasitic flatworms, which are isosmotic to the surrounding fluids of their host organisms, use their tubular excretory systems mainly to get rid of NITROGENOUS WASTES ..