Cnidarians are incredibly diverse in form, as
evidenced by colonial siphonophores,
massive medusae and corals, feathery
hydroids, and box jellies with complex eyes.
Yet, these diverse animals are all armed with
stinging cells called nematocysts. Cnidarians
are united based on the presumption that
their nematocysts have been inherited from a
single common ancestor.
The name Cnidaria comes from the Greek
word "cnidos," which means stinging nettle.
Casually touching many cnidarians will make
it clear how they got their name when their
nematocysts eject barbed threads tipped with
Many thousands of cnidarian species live in
the world's oceans, from the tropics to the
poles, from the surface to the bottom. Some
even burrow. A smaller number of species are
found in rivers and fresh water lakes.
There are four major groups of cnidarians:
Anthozoa, which includes true corals,
anemones, and sea pens;
Cubozoa, the amazing box jellies with
complex eyes and potent toxins;
Hydrozoa, the most diverse group with
siphonophores, hydroids, fire corals, and
many medusae; and
Scyphozoa, the true jellyfish.
Anthozoans are probably the most famous
cnidarians: they include the corals that build
great reefs in tropical waters, as well as sea
anemones, sea fans, and sea pens.
They also have a long and diverse fossil
record, extending back at least 550 million
years. The oldest anthozoans are probably
some of the polyp-like and sea pen-like
fossils from the Vendian (late Precambrian). A
few tens of millions of years later, in the
Cambrian period, the first mineralized coral-
like organisms appeared.
True corals of the kind living today did not
appear until the middle Triassic, at about the
same time that the first dinosaurs were
They look like your basic jellyfish, but they
can swim pretty fast, maneuver around
things, and see fairly well despite not having
a brain. Believe it or not.
In general, box jellies are similar in form to
the "true" jellyfish, known as scyphozoans.
However, it is relatively easy to tell the two
groups apart. Cubozoans have a square
shape when viewed from above.
They also have four evenly spaced out
tentacles or bunches of tentacles and well-
developed eyes. Not surprisingly, given their
squishy nature, there are not many fossil
Today, there are about 20 known species
found in tropical and semitropical waters. The
Australian stinger Chironex fleckeri is among
the deadliest creatures in the world, having
caused human fatalities.
Be careful handling this critter from Northern
Australia! Chironex fleckeri grows to about
the size of a human head, and has tentacles
up to three meters long. A big sting from this
guy can easily kill a human, with death
occurring in as little as three minutes.
There have been roughly 100 deaths due to
Chironex stings during the past 100 years in
northern Australia. However, many people
have been stung and not been killed
Contact with six to eight meters of tentacle is
necessary to deliver enough venom to kill a
Fortunately, these box jellies are in the
business of catching and eating fish and
Most hydrozoans alternate between a polyp
and a medusa stage — they spend part of
their lives as "jellyfish" which are hard to
distinguish from scyphozoan jellyfish.
A great many hydrozoans are also colonial.
Some form delicate branched colonies, while
others, known as "fire corals," form massive
colonies that resemble true corals.
Other hydrozoans have developed pelagic
(floating) colonies that are often confused
with jellyfish, but unlike jellyfish they are
composed of many individuals, all specialized
for various functions.
The "Portuguese man-o'war" and "by-the-
wind-sailors" that often wash up on beaches
are examples of these unusual colonial
The sting of Physalia is very painful to man
and can cause serious effects, including fever,
shock, and interference with heart and lung
action. When stung, carefully, pick or brush
off any visible tentacles - try not to use your
fingers - use your towel, fins, etc. Rinse with
fresh or salt water - do not use vinegar.
The nematocystic sting toxin secreted from
the tentacles of the dactylozooids, a mixture
of enzymes, is a neurotoxin about seventy-
five percent as powerful as cobra venom. The
toxins contain a complex mixture of
polypeptides and proteins.
Scyphozoans include most of the jellyfish
familiar to beach-goers; other similar
organisms are classified in the Hydrozoa and
Cubozoa, two other groups of cnidarians.
True jellyfish are graceful, and sometimes
deadly creatures. Their stings may cause skin
rashes, muscle cramps, or even death.
Jellyfish range in size from a mere twelve
millimeters to more than two meters across.
The largest is Cyanea arctica, which may have
tentacles over 40 meters long!
Despite their often enormous size, jellyfish
have no head, no skeleton, and no special
organs for respiration or excretion.
Their life cycle involves an alternation
between sesslie polyp phase and a free-
swimming medusa stage, though the medusa
stage, shown in the picture above, usually
Ctenophores, variously known as comb
jellies, sea gooseberries, sea walnuts, or
Venus's girdles, are voracious predators
Unlike cnidarians, with which they share
several superficial similarities, they lack
Instead, in order to capture prey, ctenophores
possess sticky cells called colloblasts.
In a few species, special cilia in the mouth are
used for biting gelatinous prey.