Protonephridia help for excretion and/or osmoregulation in how many of the following groups of animals?
Flatworms, roundworms, insects, rotifers, urochordates, cephalochordates
Protonephridia, or flame cells are the excretory structures in Platyhelminthes (e.g., Planaria), rotifers, some annelids and the cephalochordate (e.g., Amphioxus).