Topic summary
Protonephridia
Extracted from the Wikipedia article Nephridium.
Protonephridia
A protonephridium (proto = "first") is found in the phyla Platyhelminthes, Nemertea, Rotifera and Chordata (lancelets). They have the same anatomy as the metanephridia but with the internal ciliated funnel blocked by terminal cells: either a flame cell (if ciliated) or a solenocyte (if flagellated). Thus their tubules lack internal openings, while retaining their opening to the organism's exterior. They function in osmoregulation (ionoregulation).