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A circulatory system helps with the exchange of gases and nutrients in an animal’s body. But what if you don’t have a circulatory system? In this lesson learn how cnidarians exchange substances even without a complex internal circulation system.

Phylum Cnidaria

Phylum Cnidaria is a pretty cool group of animals. This is because cnidarians are things like corals, anemones, jellies, and hydras.

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Don’t let the peaceful look of cnidarians fool you though, because these guys are carnivorous animals that eat other animals in the surrounding water. In fact, the name Cnidaria comes from the Greek word ‘cnide’ which means nettle, and cnidarians are well-known for their stinging cells or cnidocytes.But in this lesson I want to talk to you specifically about the circulatory system of cnidarians, or rather, their lack of one.

Sea anemones are the polyp form of cnidarians.
sea anemone

What is a Circulatory System?

A circulatory system helps the body to absorb nutrients and expel waste. For example, in humans the circulatory system:

  • moves oxygen and nutrients to the cells in your body,
  • moves carbon dioxide (a waste product) to your lungs to be removed
  • moves metabolic wastes to your kidneys to be processed
  • moves hormones through your body so they can stimulate various actions
Unlike humans, cnidarians do not have a complex internal circulation system.
human circulatory system

Cnidarian Circulatory System

These exchanges sound pretty important, so you might be wondering how cnidarians can survive without a circulatory system.

Well, they still exchange gases and nutrients, they just do it a little bit differently.Cnidarian bodies are very thin and are in constant contact with the surrounding environment, so they can exchange things by direct diffusion. This is a passive process that moves particles from an area of low particle concentration to an area of high particle concentration. Genius! Instead of breathing air into their lungs like we do, they just sit around and let the water do all the work!Let’s look a little deeper to see how this works. Cnidarians come in two forms: the sessile polyp (things like anemones) and the free-swimming medusa (things like jellyfish).

  • Polyps have a mouth on the top of the body in between all of those tentacles sticking out.
  • In a medusa, the mouth is underneath in the center.

This mouth leads to the animal’s central gastrovascular cavity called the coelenteron, which digests the food for the animal as well as gives the animal that balloon-like shape. In fact, when the cnidarian closes its mouth the coelenteron will have a fixed volume of fluid, but when it opens its mouth that volume can change. This is what helps a medusa move around in the water.

The cnidarian body is pretty simple, down to two layers really. While the outside of cnidarians is lined in a layer of cells called the epidermis (similar to our skin), the coelenteron is lined with a layer of cells called the gastrodermis. Through diffusion, fluids in the coelenteron move nutrients and oxygen to other cells and at the same time remove waste products such as carbon dioxide, urea, and ammonia.But what’s really neat is that this cavity and mouth function as a two-way street.

This means that not only does food go in this way, but waste also leaves through the same opening.

The coelenteron of a jellyfish helps give it its shape.
ROLE written or published by other people
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