Excretion in Terrestrial Animals

How do land animals remove wastes while conserving water?

Land animals also face challenges. In dry environments, they can lose large amounts of water from respiratory membranes that must be kept moist. In addition, they must eliminate nitrogenous wastes in ways that require disposing of water—even though they may not be able to drink water. Figure 27–16 shows the excretory systems of some terrestrial animals.

Terrestrial Invertebrates Some terrestrial invertebrates, including annelids and mollusks, produce urine in nephridia. Nephridia (singular: nephridium) are tubelike excretory structures that filter body fluid. Typically, body fluid enters the nephridia through openings called nephrostomes and becomes more concentrated as it moves along the tubes. Urine leaves the body through excretory pores. Other terrestrial invertebrates, such as insects and arachnids, convert ammonia into uric acid. Nitrogenous wastes, such as uric acid, are absorbed from body fluids by structures called Malpighian tubules, which concentrate the wastes and add them to digestive wastes traveling through the gut. As water is absorbed from these wastes, they form crystals that form a thick paste, which leaves the body through the anus. This paste contains little water, so this process minimizes water loss.

Terrestrial Vertebrates In terrestrial vertebrates, excretion is carried out mostly by the kidneys. Mammals and land amphibians convert ammonia into urea, which is excreted in urine. In most reptiles and birds, ammonia is converted into uric acid. Reptiles and birds pass uric acid through ducts into a cavity that also receives digestive wastes from the gut. The walls of this cavity absorb most of the water from the wastes, causing the uric acid to separate out as white crystals. The result is a thick, milky-white paste that you would recognize as “bird droppings.”

FIGURE 27–16 Excretion in Terrestrial Animals Some terrestrial invertebrates, such as annelids, rid their bodies of ammonia by releasing urine created in their nephridia (left). Some insects and arachnids have Malpighian tubules, which absorb uric acid from body fluids and combine it with digestive wastes (above). In vertebrates, such as humans, excretion is carried out mostly by the kidneys (right).

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