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Chapter Three: Phylum Cnidaria
The Stinging Cell Animals – Ouch
Who are the Cnidarians?
The Cnidarians are a group of fairly simple animals who all have one thing in common, they
have stinging cells. They are all aquatic and can be found mainly in salt water although there are a few
freshwater species. They include the jellyfishes, anemones, corals and hydras.
What do Cnidarians look like?
Cnidarians have 2 basic body plans and both are radially symmetrical. They can either be a
polyp which is kind of vase-shaped or it can be a medusa which is somewhat bell-shaped. The polyp
form is the couch potato form of the animal. It cements itself to a substrate (something firm to attach
onto) and lives a sessile existence. Sessile means it is glued in place and can't move. The medusa is the
much cooler form of the two. It is free floating and has tentacles hanging down to help it move and eat.
The polyp has tentacles too but it has to wait for things to get near to it in order to feed.
Gastrovascular Cavity
Gastrovascular Cavity
How do Cnidarians Feed?
Cnidarians feed using a weapon-like cell called a cnidocyte. A cnidocyte cell is kind of like a
gun. A gun has a trigger and so does a cnidocyte. When something brushes up against the “trigger” it
causes it to fire a nematocyst which is like the bullet in a gun. The nematocyst has sharp points that
can puncture the prey's body and inject poison. The cnidarian (for example, a jellyfish) then pulls the
prey in closer so the tentacles can wrap around it and move it the mouth to get digested. Anything that
isn't digested gets expelled back through the mouth (no guts for the cnidarians).
This is what the nematocysts on
a jellyfish can do to a person!
So Do Cnidarians Have a Brain?
Not really. Cnidarians have no cephalization (concentration of nerve cells in one end) so they
don't have what we would call a brain. However, they do have something called a nerve net that allows
them to respond minimally to their environment. The nerve net is a loose web of interconnected nerve
cells distributed throughout their entire bodies. They often are clustered in rings along the outside of
the body and allow the animal to pick up stimuli like movement that they can respond to by moving
closer or farther away.
What kinds of animals are cnidarians?
There are four classes or types of cnidarians. There are hydrozoans which include some large
colonial species like the Portuguese man-of-war shown below and the much smaller fresh water hydra.
The next class contains the cubozoans. These are kind of squarish jellyfish with a tentacle
hanging down from each corner of the square. Obviously jellyfish are part of the family in a class
called Scyphozoa. And last we have the anthozoans which include the corals and sea anemones.
So which one is the cubozoan
and which is the jellyfish?
Soft coral – Can you see the
Photo Credits:
Cnidarian body plans:
Jellyfish 1:
Jellyfish sting:
Portuguese man-of-war:
Jellyfish 2: