Protonephridium

the excretory organ of most flat-worms, nemathelminths, and nemertines; it also exists among several annelids, lancelets, trochophore larvae, and the larvae of the phoronids and some mollusks.

Protonephridia are a system of simple or branching canali-culi of ectodermal origin that lie in the parenchyma or body cavity of the animal. The canaliculi empty into a main canal, which opens to the outside by means of one, two, or several pores. In trematodes and rotifers, the protonephridia open into a urinary vesicle; in acanthocephalans and priapulids, they open into the sexual ducts. Ciliate cells, known as flame cells, are located on the endogenous ends of the canaliculi. Their long cilia deeply penetrate the canalicular cavity, where their pulsating action provides a continuous current of fluid (surrounding the flame cell) through ultrathin slits in the walls of the initial section of the canaliculus and through canals leading to the excretory opening.

The protonephridia of some polychaete worms, larvae, and lancelets have a fascicle on the endogenous end of a non-branching canal. The fascicle contains numerous flask-shaped flame cells—solenocytes. (See alsoEXCRETORY SYSTEM.)