Protonephridia
Flatworms (phylum Platyhelminthes) which lack a coelom or body cavity have excretory systems called protonephridia(singular, protonephridium).the protonephridia form a net work of dead-end tubules connected to external openings. As shown in Figure 44.11,the tubules branch throughout the body. Cellular units called flame bulbs a tubule cell and a cap cell, each flame bulb has a tuft of cilia projecting into the tubule. During filtration, the beating of the cilia draws water and solutes from the e interstitial fluid through the flame bulb,
releasing filtrate into the tubule network. (The moving cilia
resemble a flickering flame, hence the name flame bulb.) The
processed filtrate then moves outward through the tubules
and empties as urine into the external environment. The
urine excreted by freshwater flatworms has a low solute concentration, helping to balance the osmotic uptake of water from environment.