Transcript Phylum Cnidaria

Phylum Cnidaria
• Notice the trend toward complexity as we go
through each phylum!
• Ny-dare-ee-ah
• The phylum Cnidaria is comprised of the
following classes:
Hydrozoa (hydras, obelia, P.M.O. War),
Scyphozoa (jellyfish) and Anthozoa (anemones
and corals).
• Comes form the Greek knide for nettle
• Cnidarians have radial symmetry
• The major advancement is the development of
true tissues.
Phylum Cnidaria
• Like Poriferans, they have two
germ layers – but these are two
true tissues (endoderm/gastroderm
and the ectoderm).
• They also have a mesoglea –
which is a jelly-like layer between
the germ layers.
• They have a nerve net and
coordinated muscular movement–
which means they can respond to
their environment.
• They also have specialized
digestive tissues.
Examples
• This phylum is exclusively aquatic.
• Most of the 10,000 species are
marine.
• Class Hydrozoa – Hydra (Freshwater)
- Portuguese Man Of War
• Class Scyphozoa - Jellyfish
• Class Anthozoa – Sea Anemones
- Coral
Man ‘o’ war
Feeding
• They have special stinging
cells called Cnidocytes.
• These contain coiled toxic
harpoon-containing capsules
called Nematocysts.
• There are thousands of these
in one tentacle of an
anemone, for example.
• Once the food is debilitated
it can be brought to the twoway mouth/anus.
http://www.youtube.com/watch?v=
Kukv0AtIVdU&feature=related
Alternation of Body Forms - Morphology
• Two main body plans are
seen
• Polyp and Medusa
• The polyp has a sessile,
tube shaped form, with
the mouth located at the
top.
• The medusa is bell
shaped and has a mouth
on the underside –
jellyfish.
• Not all species of
cnidarians have both
forms.
The Hydra was a many-headed monster slain by Heracles (AKA
Hercules). As one of his Labors, Heracles sought the Hydra's lair in
the swamps of Lerna and forced it out into the open with flaming
arrows. Wading bravely into the fray, he began to hack at the
monster with his sword. But every time he cut off one head, two
grew in its place. Eventually, Heracles called on his charioteer to
bring a torch to cauterize the Hydra's severed neck each time a head
was lopped. This prevented new heads from sprouting. And when the
final head was chopped off and buried beneath a rock, the monster
died.
Medusa was a terrible monster who had laid waste to the country.
She was once a beautiful maiden whose hair was her chief glory,
but as she dared to vie in beauty with Athena, the goddess deprived
her of her charms and changed her beautiful ringlets into hissing
serpents. She became a cruel monster of so frightening an aspect
that no living thing could behold her without being turned into
stone. All around the cavern where she dwelt might be seen the
stony figures of men and animals which had chanced to catch a
glimpse of her and had been petrified with the sight
Alternation of Body Forms
• The medusa reproduce sexually.
• They have interesting semi-permanent gonads –
cells migrate to one spot to produce gametes.
• Upon fertilization the zygote grows into a ciliated
larva – planula.
• The planula attaches to the bottom and begins the
polyp stage.
• See page 567
Alternation of Generations
• Hydra do not have a medusoid stage.
• The reproduce asexually by branching or
budding.
• Some hydra can reproduce sexually because
they are hermaphrodites.
http://www.youtube.co
m/watch?v=489CSop00
sY&feature=related
Colonies of Cnidarians
• Corals especially, can form very complex colonies.
• The diversity of corals in the ocean is amazing, and
each different type is made up of thousands of tiny
individual animals.
• Obelia, is a colonial Hydrozoan.
Ecological Role
• Symbiosis – anemones and clown fish
• Important food source for sea turtles, fish
etc.
• Examples:
• Box jellyfish – up to 3 m tentacles, kill a
grown man.
• Man’o’war