Protonephridia are also found in rotifers, some annelids,
mollusc larvae, and lancelets (see Figure 34.4). Among these
animals, the function of the protonephridia varies. In the
freshwater flatworms, protonephridia serve chchiefly in osmoregulation.
Most metabolic wastes diffuse out of the animal
across the body surface or are excreted into the gastrovascular
cavity and eliminated through the mouth (see Figure 33.10).
However, in some parasitic flatworms, which are isoosmotic
to the surrounding fluids of their host organisms, the main
function of protonephridia is the disposal of nitrogenous
wastes. Natural selection has thus adapted protonephridia to
different tasks in different environments.