Flatworms, ribbon worms, and some other invertebrates have a simple, tubular excretory system. Specialized cells called flame cells draw in fluid wastes from the surrounding body cells, and bundles of cilia drive the fluid into a branched system of excretory ducts that open on the animal’s surface.

This type of excretory system works only in aquatic animals, which can use the surrounding water to flush out the excretory ducts. In insects, the excretory system consists of slender tubules anchored at the anterior end of the hindgut. These Malpighian tubules collect wastes and water from the body fluids and discharge them into the hindgut, where much of the water is reabsorbed. The final excretory product is mainly uric acid crystals, which pass out with the feces.

The kidneys of vertebrates are derived from much simpler organs called nephridia—tubes that open to the outside of the body. In an earthworm, each body segment contains two nephridia. Fluid wastes enter each nephridium through a funnel in the body cavity of the segment in front of it.

The waste passes through a coiled tube bordered lined by a transport epithelium, and is stored in a collecting bladder, which opens to the outside through a pore.

A network of capillaries surrounding the coiled tube reabsorbs nutrients through the transport epithelium, leaving the nitrogenous waste in the tubule to be excreted as urine.

As we discuss excretion in more complex animals, you’ll see that the same principles are involved. The segmental arrangement of the nephridia seen in earthworms and other invertebrates gradually evolved into the kidneys of vertebrate animals.

If you’ve ever gutted a fish, then you’ve seen fish kidneys. Kidneys of fish and amphibians are longer and thinner than those of other vertebrates. The main function of fish kidneys is osmoregulation. The excretion of nitrogenous waste in fish takes place through the gills.

The kidneys of reptiles, birds, and mammals are not segmental. Each animal has only a single pair of kidneys. Vertebrate kidneys also remove waste only from the blood supply—not from the body cavity. These animals all have different excretory systems. Match the animal to its excretory system and then click Submit.

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The flatworm has a system of ducts and flame cells to excrete waste products. The insect has Malpighian tubules that join to the hindgut.

Earthworms have nephridia in their body segments, and vertebrates have compact kidneys. that consist of structures called nephrons to filter waster products.

Now that we’ve seen the excretory systems of a number of different animal groups, we’ll go on to look in detail at the mammalian kidney.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education