Cnidaria Nervous System

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

In this lesson, we'll take a look at the simple and elegant nervous system of the Phylum Cnidaria. Cnidarians include hydras, sea jellies or jellyfish, sea anemones, corals, and box jellies. Their nervous system allows them to sense their environment, move through the water, and capture prey.

Simple Creatures

Cnidarians are a fascinating -- and sometimes odd-looking -- group of simple but elegant animals. The Phylum Cnidaria (pronounced nid-AIR-ee-ah) are sea jellies or jellyfish, sea anemones, corals, and box jellies. They are infamous for their stinging cells and their simple body plans. They are, in essence, living, swimming nerve nets. How can something so biologically simple still continue to be an animal? The answer ties back to the way in which their nervous system is structured.

Sea nettle jellies, also known as sea nettle jellyfish.
sea nettle jellyfish

Let's take a look at the nerve net of Phylum Cnidaria, and discuss their specialized cnidocyte cells that help them capture prey.

Nerve Net

What is a nerve net? Just like a regular net of webbing, a nerve net is an expansive thing, stretching across great distances. In the cnidarian body, the nerve net serves as a sensory locator; neuron cells stretch all around the animal's body and allow the cnidarian to detect chemical changes, to capture prey, and to move in response to a stimulus. This kind of expansive nerve net is also known as a diffuse nerve net.

A diffuse nerve net is different from the radial, or circular, nerve net that is found in other radially symmetric animals, such as sea stars. Interestingly, the cnidarian nerve net doesn't allow the cnidarian to detect the source of a stimulus, and these animals will move in the same coordinated response pattern no matter where the stimulus is located.

Occasionally, nerve nets in cnidarians are bundled into groupings known as ganglia (singular: ganglion). While these bundles are not true brains, they serve as an intermediary point between sensory neurons that process environmental signals and other parts of the cnidarian body.

Cnidocytes: Specialized Cells

Cnidocytes are specialized cells that act as tiny harpoons, blasting outward to poison or capture prey. They are responsible for the nasty stings of jellies.

A cnidarian nematocyst, or cnidocyte.

Cnidocytes fire a variety of different injection probes into prey animals or as a defense mechanism. Each cell is equipped with a 'finger' (in red above) that inverts inside out to project a barb into the target. The barb is often filled with poison and is loaded at the end of a coiled thread inside the cell. A 'lid' (in blue) covers the opening, or operculum, of the cnidocyte cell at rest. Cnidocytes - literally, 'stinging cells' - are also called nematocysts, or 'thread cells'.

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