The Circulatory System of Platyhelminthes

Instructor: Jeremy Battista
Breathing is necessary for life, right? What about our more simplistic animals? Do they need to breathe? Do they need blood? Let's look at the circulatory system for platyhelminthes and discuss how it allows them to live.

Platyhelminthes: A Background

Platyhelminthes might be a weird sounding word, but it is the name of a phylum in the animal kingdom. A phylum is the second least specific classification in taxonomy or the classification of living things. Platyhelminthes are the flatworms, animals that are rather simple in scope but much more complex than some of the phyla that existed before them.

A flatworm.
Flatworm

These animals are quite literally, flat. They do not look like the typical earthworm that you might picture when you hear the word 'worm.' Instead, they look similar to those round worms but are flattened out, appearing like they had been run over by a steam roller in an old-time cartoon.

Common Features

There are four classes of flatworms; three are parasitic and one is not. Given this, you can assume that the parasitic classes have different mouthparts than the other class. Even so, most flatworms have similar features as they are a phylum unto themselves. Their nervous systems, digestive systems, etc. would all be fairly similar with minor differences. Their circulatory systems would also be fairly close to one another, which we will go into further.

Circulatory System

When you hear the words 'circulatory system,' you might picture the heart, veins, arteries and lymph nodes as a human would have. Remember that we are talking here about animals that are much more simplistic than humans and lack the dimensions that we have as humans.

Flatworms do not have a circulatory system in the usual sense. There are no veins, lymph nodes, or arteries. Instead, because they are fairly small in size and are so flat, flatworms are able to breathe through their 'skin,' which is really just integument, a moist outer covering. This 'breathing' allows the animal to exchange oxygen with the outside, bringing it into the animal and then allowing it to be used wherever needed. There is no need for a special circulatory system as a human has because the flatworm doesn't breathe with lungs and doesn't need to transport oxygen around its body. The flatworm simply diffuses oxygen via its skin. This occurs wherever it may need the oxygen. For example, it more oxygen is needed in the anterior portion of the animal, the oxygen will diffuse there. If more is needed in the middle area of the animal, more oxygen diffuses in that region.

A diagram of diffusion. Notice it starts with a higher concentration and ends up with an even amount on both sides of the red line.
Diffusion

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