Why Don't Cnidaria Have a Coelom?

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will discuss the characteristics of animal guts, and what ways Cnidarian guts are similar to or different from other animal guts. We will discuss the structure, operation, and functions of the Cnidarian body cavity.

The Animal Gut

Ever since you were nothing but a tiny ball of cells, you had the beginnings of a gut. In fact, you can think of yourself as a giant tube because your guts run all the way through you. But not all animals are tubes! A jelllyfish, for example, is shaped more like a cup. But even jellyfish must have a gut, since there has to be somewhere for food to go! Most animals' guts are called coeloms, but Cnidaria do not fall into this category. Let's talk about why.

The Coelom

When you were a very young fetus, you-- like most multicellular animals--developed into three basic tissue layers:

  1. the ectoderm (outside layer)
  2. the mesoderm (middle layer)
  3. the endoderm (inside layer)

As you developed further, much of your ectoderm became your skin. Much of your mesoderm became your muscles and internal organs, while much of your endoderm became your guts. Your three tissue layers make you a 'triploblast'.

A few animals, such as jellyfish and related creatures, are diploblasts. That means that they developed from only two basic tissue layers: the ectoderm and the endoderm.

A coelom is the gut of tripolblasts. It is a completely encased, fluid-filled body cavity lined by mesodermic tissue. You will often see literature classifying animals as having or not having a coelom. The majority of animal phyla have coeloms.

However, no diploblast has a coelom, because they do not have the mesoderm tissue to make a proper coelom. Let's talk about what one group of diplobasts do have.

Cnidaria Gut

Cnidaria are a phylum of diploblasts, meaning that they do not have a coelom. Cnidaria consist of around 10,000 species of aquatic animals, including jellyfish, sea anemones, hydra, box jellies, and corals.

The defining feature of the Cnidaria are cnidocytes, fascinating, harpoon-like stinging cells that are most often used for capturing prey. A jellyfish's sting comes from its cnidocytes. Though we often think of Cnidaria as simple creatures, scientists have refered to cnidocytes as the most complex cell type in the animal world.

Think of Cnidaria as one giant gut with three parts.

  1. Their outer tissue layer, the epidermis, contains all the nerve and muscle cells, and most of the cnidocytes.
  2. The middle part is a mucus-like secretion called the mesoglea that separates the two tissue layers. It's that clear, wobbly, Jell-O like stuff you see in jellyfish on the beach.
  3. The inner layer, the gastroderm, is what lines their gut cavity. It contains gland cells that secrete digestive enzymes, and may have a few cnidocytes to help overpower any prey that is still struggling.

Incomplete and Complete Digestive Tracts

Is your digestive tract shaped more like a pipe or a balloon?

In a way, your digestive tract is shaped more like a pipe because you have a 'hole' that goes all the way through you. In that sense, chordates (the phylum of animals to which you belong) have a complete digestive tract. You eat at one end and excrete from the other end. Some simple animals like jellyfish are shaped more like balloons, eating and excreting through the same opening. These animals, like the Cnidaria, have an incomplete digestive tract.

Having an incomplete digestive tract comes with some challenges. For instance, you have to be completely done digesting your last meal before you eat again. Also, you can't be as selective with your enzymes- you can't choose to use some enzymes for food that you are just beginning to digest and different enzymes for waste products.

Other Uses for a Cnidarian Gut

What can you do with a Cnidarian gut besides eat?

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