Gastrovascular Cavity: Definition & Explanation Video

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  • 0:05 What is a…
  • 0:36 Animal Phyla
  • 1:06 Basic Structure and Function
  • 1:36 Cnidaria
  • 2:45 Platyhelminthes
  • 3:41 Lesson Summary
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Lesson Transcript
Instructor: Nicholas Gauthier
Primitive animals have body plans that are very different from ours. Animals such as jellyfish, corals and flatworms have a much simpler way to handle the digestion of food, as well as the circulation of nutrients around the body. Learn about the structure and function of the gastrovascular cavity in these organisms.

What Is a Gastrovascular Cavity?

The gastrovascular cavity is a structure found in primitive animal phyla. It is responsible for both the digestion of food and the transport of nutrients throughout the body. The cavity has only one opening to the environment. Food goes in and waste comes out that same opening, making it a two-way digestive tract.

By contrast, organisms that have a mouth on one end with an anus on the other end have a one-way digestive tract, called an alimentary canal. Food goes in the mouth, while waste comes out the anus.

A gastrovascular cavity (left) has only one opening, while an alimentary canal (right) has two.
Gastrovascular Cavity vs. Alimentary Canal

Animal Phyla

There are two phyla under Kingdom Animalia that possess a gastrovascular cavity. The first is Cnidaria, which includes jellyfish, corals, hydras and sea anemones. Cnidarians show radial symmetry, which means that they are similar when transected across the middle. The second is Platyhelminthes, which includes flatworms, such as Planaria, and tapeworms. Platyhelminthes show bilateral symmetry, which means they have identical halves when transected longitudinally.

Radial symmetry (left) vs. bilateral symmetry . Each line segment divides the organism into roughly equal halves. Note that in bilateral symmetry, only one such division is possible.

Basic Structure and Function

The gastrovascular cavity is essentially an empty space in the organism that is surrounded by tissue. Food is brought into the cavity. The tissue surrounding the cavity secretes enzymes to enable digestion. Nutrients are then absorbed directly by the cells surrounding the cavity and distributed to the rest of the organism through diffusion. The organisms possessing a gastrovascular cavity are very simple and made of only a few tissues. Therefore, there is no need for a true circulatory system.


The gastrovascular cavity in Cnidaria is surrounded by an inner tissue layer called the gastrodermis. The outside of the organism has a tissue layer called the epidermis. Between these two tissue layers is a gel-like region called the mesoglea.

The mesoglea is made of mostly water with some fibrous tissue. It serves as a sort endoskeleton for the organism. Phagocytic cells called amoebocytes wander through the mesoglea, devouring invading organisms. In some of the more advanced cnidarians, the mesoglea is more developed, having muscle and nerve tissue.

It should be noted that cnidarians come in two general body plans - the medusa form found in jellyfish, and the polyp form found in corals, hydras and sea anemones.

The medusa and polyp versions of Cnidarian body plans. Used with permission by Laura Smith.

Sea anemone, which has a polyp form. This one is brooding.
Sea Anemone

Sea nettles, which have a medusoid form.
Jellyfish, Sea Nettles

Cnidarians, despite being cup-shaped, are essentially flat organisms that are folded into the medusoid and polyp forms, with the addition of tentacles and nematocysts, or stinging cells. The thin nature of the bell or vase, consisting of only the gastrodermis, mesoglea and epidermis, allows for sufficient respiration by diffusion.

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