Cnidarians

Where is your Mouth?

These creatures come in two forms, a polyp, which is sessile or attached to a surface, and a medusa, which is planktonic.  Although free-floating, planktonic organisms are unable to generate willful motion and are at the mercy of the currents.  Despite this inability to swim, both the medusa and polyp forms of cnidarians are carnivorous. Although they’d prefer to eat small fish, crustaceans, zooplankton and even other jellyfish, they will resort to grazing on phytoplankton if their carnivorous fare is unavailable.

The mouthThe opening of the digestive tract where food enters and mastication begins. of the polyp face upwards, like in a anemone, and faces downwards as in the jellyfish.  Think of their food source, for the polyp, attached to a rock, food comes raining down from above.  For the jellyfish, it gets tangled in their many stinging tentacles.

Radial Symmetry

Although we may not realize it, cnidarians have radial symmetry, unlike those asymmetrical poriferans.  This means that no matter which way you cut it, you get two equal halves.  Cnidarians also have a hydrostatic skeleton.  This is a waterThe universal solvent essential for life. filled skeleton kind of like a water balloon.

Glow-in-the-Dark Sea Creatures

Cnidarians have a nervous systemThe organ system that controls body functions using electrical and chemical signals. that is a loosely organized net of nervous tissue.  It allows some very basicA solution with a pH above 7, having a lower concentration of H⁺ ions. sensory and control mechanisms. There is a group of organisms called Ctenophores, casually known as comb jellies.  These are the jellyfish that are perfectly round and hard, like flying saucers.  They have a nerve net that is phosphorescent.  As they break apart on the beach, pieces of the wash up and glow in the moonlight.  All I remember is these are the ones we used to throw at each other’s faces when we were surfing (I grew up at the beach).