Invertebrate Excretion Systems and Animal Waste Products

Excretion Organs in Invertebrates

In primitive animals, excretion occurs externally by diffusion across the body surface.

Protonephridia

Found in flatworms, these are highly branched tubes whose inner ends terminate in a cell with flagella, known as a flame cell, while the outer end is an excretory pore. Flame cells are located among intercellular liquids. Waste substances pass through diffusion to the cells and from there to the tubules, where the liquid is expelled, aided by the movement of the flagella.

Metanephridia

Found in annelids and molluscs, this system consists of a long tube with one internal funnel-shaped end called nephrostomes, which possess cilia. The motion of these cilia drives the liquid towards the inside of the tube. Along the tube, necessary substances are reabsorbed, leaving an aqueous liquid containing only the waste, which is removed through an excretory pore called the nephridiopore.

Antennal and Maxillary Glands

These are found in crustaceans and are named according to whether they are located at the base of the antennae or the maxilla. In decapods, these are known as green glands or antennal glands. The glands are paired and surrounded by blood capillaries; they consist of a glandular chamber, a muscular bladder, and an external excretory pore.

Malpighian Tubules

Found in insects, millipedes, and some spiders, these are thin tubes distributed throughout the body. They have one blind end and another end that opens to the intestine, where they discharge waste products.

Animal Excretion Products

Excretion products are derived from nitrogen, minerals, water, and CO2.

Nitrogenous Derivatives

These are produced during the degradation reactions of proteins and nucleic acids:

• Ammonia: Highly toxic, although it is diluted in water. It is eliminated by ammoniotelic animals, such as aquatic worms, bony fishes, crustaceans, amphibian larvae, and other aquatic invertebrates.
• Uric Acid: This can form in the liver from ammonia and other nitrogenous substances. Uricotelic animals are adapted to live in dry environments; these include reptiles, insects, and birds.
• Urea: This forms in the liver from nitrogenous amino acids and CO2. It is found in ureotelic animals, such as cartilaginous fishes, amphibians, and mammals. It is less toxic and is expelled as an aqueous liquid dissolved in water and salts.

Minerals

Minerals are excreted to regulate and maintain an adequate quantity and concentration within the internal environment.