Nephridium

These ciliated tubules pump water carrying surplus ions, metabolic waste, toxins from food, and useless hormones out of the organism by directing them down funnel-shaped bodies called nephrostomes.

A protonephridium (proto = "first") is found in the phyla Platyhelminthes, Nemertea, Rotifera and Chordata (lancelets).

Because of this, pressurization drives waste fluids from the inside of the animal, and they are pulled through small perforations in the terminal cells and into the protonephridium.

The perforations in the terminal cell are large enough for small molecules to pass, but larger proteins are retained within the animal.

Protonephridia likely first arose as a way to cope with a hypotonic environment by removing excess water from the organism (osmoregulation).

An organ of excretion in flatworms: a hollow cup-shaped cell containing a bunch of cilia or flagellum, whose movement draws in waste products and wafts them to the outside through a connecting tubule.