Download Phylum Cnidaria

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General Characteristics
• They are radially
symmetrical; oral end
terminates in a mouth
surrounded by tentacles.
• They have 2 tissue layers
• Outer layer of cells - the
epidermis
• Inner gastrodermis, which
lines the gut cavity or
gastrovascular cavity
(gastrodermis secretes
digestive juices into the
gastrovascular cavity)
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General Characteristics
 In between these tissue
layers is a noncellular
jelly-like material called
mesoglea
 Manubrium – tubelike
structure that hangs
from the medusa’s oral
surface (mouth is found
at the end).
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Body Structure
 Hydrostatic skeleton – water or body fluids confined in
a cavity of the body and against which contractile
elements of the body wall act.
 Statocyst – a small sac surrounding a calcium
carbonate concretion. “Stone” moves in response to
the pull of gravity, initiating nerve impulses.
 Oral disc – area surrounding mouth. Contains
openings which permit water to circulate.
 Acontia – “Threads” found at the end of mesenterial
filaments which contain cnidocytes. Subdue live prey
in the gastrovascular cavity.
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Cnidarian Body Plan
Polyp Form
Medusa Form
• Tubular body, with the
mouth directed
 Bell-shaped or umbrella
upward.
• Around the mouth are
a whorl of feeding
tentacles.
• Only have a small
amount of mesoglea
• Sessile
shaped body, with the
mouth is directed
downward
• Small tentacles, directed
downward.
• Possess a large amount of
mesoglea
• Motile, move by weak
contractions of body
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Cnidarian Body Plan
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Tentacles
 Prey capture is enhanced
by use of specialized
stinging cells called
cnidocytes located in the
outer epidermis.
 Each cnidocyte has a
modified cilium - cnidocil,
and is armed with a
stinging structure called a
nematocyst.
 The undischarged
nematocyst is composed of
a long coiled thread
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Tentacles
 When triggered to release,
either by touch or
chemosensation, the
nematocyst is released
from the cnidocyte and the
coiled thread is everted
 Some nematocysts
function to entangle the
prey; others harpoon prey
and inject a paralyzing
toxin
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Cnidarian Life Cycle
Stage 1 – Adult Medusa Stage 2 – Planula larva
Stage 3 - Polyp9
Reproduction
• One of the most amazing adaptations is the ability of some
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cnidarians to regenerate lost parts or even a complete body
Asexual reproduction is common with new individuals
being produced by budding
Sea anenomes engage in a form of asexual reproduction
called pedal laceration
Cnidariand are dioecious
Fertilization is external, with the zygote becoming a
elongated, ciliated, radially symmetrical larva - planula
larva
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Nutrition
 Cnidarians are carnivores with hydras and corals
consuming plankton and some of the sea anenomes
consuming small fishes
 They use they tentacles to capture prey and direct it
toward the mouth so that it can be digested in the
gastrovascular cavity via secretions from gland cells
(extracellular digestion); some food is phagocytized by
special cells and digestion occurs intracellularly
 The gastrovascular cavity exists as 1 opening for food
intake and the elimination of waste
 There is no system of internal transport, gas exchange or
excretion; all these processes take place via diffusion
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Movement
 The cnidarian body is capable of some kind of
coordinated movement
 Both the epidermis and the gastrodermis possess
nerve cells arranged in a loose network - nerve net
(plexus), which innervate primitively developed
muscle fibers that extend from the epidermal and
gastrodermal cells
 Stimulus in one part will spread across the whole body
via the network
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Response to Stimuli
 No cephalization or nervous system
 Nerve net around mouth
 Sensory Cells
 Chemoreceptors (chemicals)
 Thigmoreceptors (touch)
 Photoreceptors (light) Ocelli (eyespots)
 Statocysts (balance)
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Class Hydrozoa
 Includes the solitary freshwater hydra; most are
colonial and marine
 Typical life cycle includes both asexual polyps and
sexual medusa stages; however, freshwater hydras and
some marine hydroids do not have a medusa stage
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Solitary Hydras
 Freshwater hydras are found in
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ponds and streams occurring on the
underside of vegetation
Most possess a pedal disc for
movement, mouth, hypostome
surrounded by 6-10 tenetacles
Mouth opens to the gastrovascular
cavity
The life cycle is simple: eggs and
sperm are shed into the water and
form fertilized eggs; planula is by
passed with eggs hatching into
young hydras
Asexual reproduction via budding
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Colonial Hydrozoans
• Possess a skeleton of chiton that is
secreted by the epidermis
• All polyps in the colony are usually
interconnected
• Two different kinds of individuals
that comprise the colony: feeding
polyps or gastrozooids (C) and
reproductive polyps or gonozooids
(B)
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Other Hydrozoans
 Portuguese man-of-war:
 Single gas-filled float
with tentacles
 Tentacles house the
polyps and modified
medusae of the colony
 http://www.youtube.com/
watch?v=xTgLTbXJrfM
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Class Scyphozoa
 True Jellyfish
 The medusae are large and
contain massive amounts of
mesoglea
 The differ from the hydrozoan
medusa in that the lack a
velum
 Possess four gastric pouches
lined with nematocysts; these
are connected with the mouth
an the gastrovascular system
 http://www.youtube.com/watc
h?v=u0I-3wkH37w
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Class Anthozoa: Corals
• Exclusively marine; there is no medusa stage
• At one or both ends of the mouth is a ciliated groove
called the siphonoglyph; generates a water current
and brings food to the gastrovascular cavity
• Possess a well developed pharynx
• The gastrovascular cavity is large and petitioned by
septa or mesenteries; increase surface area for
digestion or support
• Edges of the septa usually have threadlike acontia
threads, equipped with nematocysts and gland cells
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Coral Video
 http://www.youtube.com/watch?v=-O-m_XIrA1k
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Class Anthozoa
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Class Anthozoa
• Solitary anthozoans include sea
anemones
• Most anthozoans are colonial (e.g.
corals) and secrete external
skeletons composed of calcium
carbonate.
• Corals obtain much of their energy
from microscopic photosynthetic
green algae (zooxanthellae) or
dinoflagellates that live
symbiotically inside the cells of the
coral
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Phylim Ctenaphora
 Contains Comb Jellies
 Characteristics of
Ctenophora:
 Biradial symmetry
 Monoecious
 Gastrovascular cavity
 Rows of ciliated
bands, called “comb
rows”
 No nematocysts;
Adhesive structures
known as colloblasts
http://www.youtube.com/watch?v=39qe0E
ajbIA
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Comb Jellies
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