Introduction to Cnidaria - mrstephens

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Introduction to Cnidaria - mrstephens Powered By Docstoc
					   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
    poison.
   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
    evolving.
   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
    cubozoans known.
   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
    person.
   Fortunately, these box jellies are in the
    business of catching and eating fish and
    crustaceans.
   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
    hydrozoans.
   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
    predominates.
   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
    stinging cells.
   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.

				
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posted:12/7/2012
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