Flame cells are primitive excretory organs found in

________.

a. arthropods

b. annelids

c. mammals

d. flatworms

Before the evolution of complex excretory organs, simple processes were used by invertebrates and bacteria to eliminate metabolic waste. Flame cells, contractile vacuoles, Malpighian tubules, and nephridia are some of the primitive excretory organs.

Arthropods use the Malpighian tubule system for removing waste from their bodies. The Malpighian tubules are finger-like structures located between the midgut and hindgut that collect waste from the hemolymph. Important nutrients are reabsorbed and uric acid crystals are eliminated from the hindgut. Hence, option a is incorrect.

The excretory organs in annelids are nephridia which have a funnel-like opening lined with cilia. The fluid enters through this opening and passes through the tubule. Reabsorption of important nutrients takes place as the fluid passes through the tubules and the waste is excreted at the end of the tubules via the nephridiopore. Hence, option b is incorrect.

Mammals have a pair of kidneys that perform blood filtration to remove waste products in the form of urine. Hence, option c is incorrect.

Flame cells or solenocytes are excretory organs found in flatworms. Flame cells collect nitrogenous waste in the form of ammonia from the interstitial fluid. The bundle of cilia present in the flame cells pushes the waste into the tubules and is removed from the body via the excretory pore. Hence, option d is correct.