Excretory Structures Across the Animal Kingdom

Before the vertebrate kidney evolved, the animal kingdom experimented with several very different excretory arrangements, and simpler organisms still rely on them today. In unicellular organisms such as Amoeba and Paramecium, the contractile vacuole does the job — it collects excess water and waste and periodically discharges it out of the cell. Sponges have no dedicated excretory organ at all; waste simply diffuses into the surrounding water current and is carried out through the sponge's osculum.

Once animals evolved bilateral symmetry, true excretory organs appear, generally built as a simple or branching tube opening to the exterior through a pore called a nephridiopore. Two major types of nephridia are recognised. Protonephridia are networks of dead-end tubules called flame cells — found chiefly in animals that lack a true body cavity, such as flatworms (Platyhelminthes), and also in rotifers, some annelids and Amphioxus. Metanephridia, by contrast, are unbranched, coiled tubes that open into the body cavity through a funnel-shaped nephrostome; body fluid enters through the nephrostome and is discharged through the nephridiopore — earthworms are the classic example. …