Protonephridia are generally found in basal organisms such as flatworms

. Protonephridia likely first arose as a way to cope with a hypotonic environment by removing excess water from the organism (osmoregulation). Their use as excretory and ionoregulatory structures likely arose secondarily.

What animals contain Protonephridia?

The simpler, more primitive protonephridia, found in flatworms, ribbon worms, and rotifers, are usually scattered among the other body cells. More advanced, segmented invertebrates, such as earthworms, possess the more complex metanephridia, usually arranged in pairs.

Which annelid has Protonephridia?

Certain meiofaunal annelids , especially myzostomids and certain phyllodo- cidan annelids , including all species of the Phyllodocidae, Nephthyidae, Glyceridae, and Gonadiidae, possess segmental protonephridia instead of segmental metane- phridia . ...