An Introduction to Zoology: Chapter 11

Respiration, Circulation, and Excretion:

In free-living platyhelminths, the gut is highly branched and extends throughout the body. This means that no body cell is more than a millimeter or so away from the gut, and so can easily absorb digested food from it. Similarly, the thin, flat body of a platyhelminth means that no body cell is far from the surrounding air or water, so oxygen can diffuse from the air into body tissues while the CO2 produced as a metabolic waste product can easily diffuse out of the animal’s body tissues and into the surrounding air or water.

All of this means that platyhelminths need neither circulatory systems to transport substances within the body nor respiratory systems to move oxygen and carbon dioxide in and out of body tissues. On the other hand, the platyhelminths are the first animals we have discussed that have anything resembling an excretory system.


In most animals, an excretory system has two functions – excretion and osmoregulation. Excretion is the process in which the body rids itself of potentially-toxic metabolic wastes. In platyhelminths, excretion is rarely a problem, since metabolic wastes such as ammonia can simply diffuse out of animals’ bodies. But osmoregulation is an entirely different matter.



If an organism’s cells have a higher solute concentration than does the water it lives in, then the water is said to be hypotonic to the animal’s cells. Of course, if the animal’s cells have a higher solute concentration than does the surrounding water, that means their water concentration is lower. This is a problem for any animal that lives in fresh water.

If cells are surrounded by a hypotonic solution, because the cells have a higher solute concentration (and, therefore, a lower water concentration) than does the solution, water tends to spontaneously diffuse into the cells from the solution. So, freshwater animals have the problem that their cells are constantly absorbing water. If the animal has no way to get rid of this excess water, its cells will absorb water until they burst, killing the animal.

So, freshwater animals must have some means of expelling excess water. The process of regulating the water (or, more precisely, the salt) content of body tissues is known as osmoregulation, and it’s literally a matter of life-and-death for freshwater animals.


If an animal’s cells and the water it lives in have an equal solute concentration, then the water and the animal’s cells are said to be isotonic. Many (but by no means all) marine animals have body tissues that are isotonic to the surrounding seawater. If an animal’s cells are isotonic to the surrounding water, then they neither gain nor lose water. Such an animal does not have to osmoregulate.


Many marine animals must be capable of osmoregulation, however, because their cells have a lower solute concentration than does the surrounding seawater (and, therefore, a higher water concentration). When this is the case, the seawater that the animal lives in is said to be hypertonic to the animal’s tissues. An animal’s cells will lose water to a hypertonic solution, so many marine animals must constantly drink, in order to replace fluid lost to the surrounding sea. In other words, ironic as it seems, one of the biggest problems faced by many marine organisms is dehydation.