Steph has a PhD in Entomology and teaches college biology and ecology.
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.
When you were a very young fetus, you-- like most multicellular animals--developed into three basic tissue layers:
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 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.
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.
What can you do with a Cnidarian gut besides eat?
You think of your gut cavity as having primarily one purpose: to digest food. But having a balloon-shaped body like a Cnidarian can be useful for other reasons as well. Some Cnidarians are sessile, but mobile Cnidarians contract their gut cavity to swim, in a motion similar to jet propulsion. Some sessile Cnidarians, like sea anemones, close their mouths when they are not eating, filling with water that functions as a sort of hydrostatic skeleton.
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The three basic tissue layers in most animal phyla are the ectoderm, mesoderm, and endoderm. An animal that has all three layers is called a triploblast. Some animals though, like the Cnidaria, are diploblasts, and do not have a mesoderm.
A coelom is a fully-encased, fluid-filled body cavity (gut) lined with mesodermic tissue. Most complex, multicellular animals have a coelom. Cnidarians are not considered to have a coelom because they are diploblastic, so they don't have any mesodermic tissue.
Cnidaria are a phylum consisting of aquatic animals like jellyfish, anemones, and corals. Cnidaria have cnidocytes, specialized stinging cells. Cnidaria have only two layers of cells separated by a gooey secretion called a mesoglea. Lining their digestive cavity is the gastroderm, which secretes digestive enzymes.
Cnidarians have an incomplete digestive tract, which means that they eat and excrete from the same opening. Chordates (like you) have a complete digestive tract, meaning that you eat and excrete from different openings. An incomplete digestive tract means that an animal can't eat more food until it is done digesting its last meal.
Cnidaria also use their coeloms to swim and even to form a hydrostatic skeleton.
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