Cnidaria Gas Exchange System

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

The phylum Cnidaria is diverse and fascinating. One thing that makes them so interesting is how gas exchange occurs in their bodies. Learn more about Cnidarian gas exchange and how it is different from many other animals.

The Phylum Cnidaria

Imagine if you could breathe through your skin instead of your mouth - if nothing else it would certainly make for some interesting party tricks!

The name Cnidaria may not immediately ring a bell, but I'm pretty sure you've already heard of these guys. This is because the phylum Cnidaria includes things like corals, jellies, and hydras. The phylum is made up of some 10,000 species, all of which are found in either fresh- or saltwater environments.

Some are harmless, while others can cause you some serious discomfort if you happen to come across their stinging cells. Cnidarians come in two forms: the sessile polyp and the free-swimming medusa. For example, corals are sessile, while adult jellies are free-swimming.

The free-swimming medusa (left) and sessile polyp (right) stages of Cnidarians.
cnidarian poly and medusa

Cnidarians may look very different from you and me, but because we are all animals we actually have some basic similarities. We have to reproduce to create new generations of offspring, we have to eat to give our bodies the nutrition they need to live and grow, and we have to 'breathe', which involves the exchange of gases. And this gas exchange is the focus of our lesson today.

The phylum Cnidaria contains some of Earth
lionsmane jelly

Gas Exchange in Animals

Before we get into how Cnidarians go about gas exchange, let's take a moment to review why this gas exchange is so important. Think about human gas exchange for a minute: you breathe in oxygen and breathe out carbon dioxide. You do this because you need oxygen for many cellular processes, and carbon dioxide is a waste product so you need to expel this gas.

Well, other animals need to do the same thing in order to drive their cellular processes. Some do it through lungs like us, others through their gills like fish and sharks. Others exchange gas through means such as passive exchange through their skin like frogs. And while they don't necessarily breathe gases in and out the same way we do, it's still an important process for all animals in order to remain happy and healthy.

Gas Exchange in Cnidarians

Cnidarians don't have lungs, and even though they live in aquatic environments they don't have gills either. So they have to exchange 'good' and 'bad' gases a little bit differently. Instead of breathing, gas exchange in Cnidarians occurs through direct diffusion. This is the process by which particles move from a place where the particle concentration is high to a place where the concentration of those particles is low. Amazingly, diffusion occurs inside your body as well, and is the mode of transport for many molecules across your cell membranes.

In diffusion, particles will naturally move from an area of high concentration to an area of low concentration.
diffusion

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