Although cnidarians vary greatly in appearance,
they do have common characteristics that seperate them from other groups.
A common characteristic that has given this group its name, are its stinging
cells (cnidoblasts). When touched they can discharge a barbed thread that
is connected to a venom sac.
Cnidarians use their stinging cells to incapacitate their
prey. Large cnidarians like jellyfish and anemones are predators that
can attack large prey - e.g. fish and hermit
crabs. Probably the most important source of food for cnidarians are
that hover in the water.
Stinging cells can even be used for defence. However, most
stinging cells are of insignificant strength to cause discomfort to man,
but there are a few exceptions, like the lion´s
mane jellyfish. A few specie can be very venomous. The portuguese
man-o-war, that is found in the warmer regions of the Atlantic can
cause very painful injuries. Along the beaches of e.g. Australia, signs
are often posted warning for a special type of cnidarian, the box
jellyfish. Injuries from box jellyfish can be lethal if medical
attention is not acquired in time. In Swedish waters only the stinging
jellyfish can cause discomfort.
Stinging cell before and after ejection. The brush is the "trigger"
that causes the stinging cell to explode and cast out the barbed thread.