Download PHYLUM CNIDARIA Body Forms

3/24/2017
PHYLUM CNIDARIA
Greek root “cnido” means “stinging nettle”
Thin-walled, soft-bodied animals with stinging tentacles
General characteristics
About 11, 000 species including jellies, corals, sea
anemones
- All members are aquatic (most are marine)
- United by stinging tentacles
- Tissue level of organization
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More complex than sponges but very simple
Have true tissues but few primitive organs
Radial symmetry (as adults)
Many are colonial (such as Portuguese Man O’
War)
Most are sessile, some are planktonic, few are
nektonic
Body Forms
Many Cnidarians are polymorphic which means
they have two or more separate body forms with
alternation between forms
The same species has two distinct forms:
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POLYP
– cylindrical, vase-shaped
- mouth faces up
- sessile
- asexual reproduction stage
-Corals (left), hydras, &
sea anemones exist in
polyp form as adults
MEDUSA - bell-shaped
- mouth down
- free-swimming/planktonic
- sexual reproduction stage
- jellies & Portugese man-of-war as
adults
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Cells and Tissues
Cnidarians have 2 tissue layers (most animals have 3):
epidermis – protective outer layer contains
stinging cells
gastrodermis – inner layer forms gastrovascular
cavity- cells in this layer aid in
digestion & circulation
* Between the two layers of tissue is a thick jelly layer
called mesoglea. Mesoglea is very thick in jellies, it’s really
why they are so jelly-like. Meloglea is not true tissue.
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Movement
lack true muscle cells, but have special
epidermal cells that allow tentacle movement
medusa can open and close like an umbrella,
allowing it to move by “jet propulsion” when
water is forced out of the medusa
Feeding and Digestion
- All are carnivores
- Mouth is surrounded by tentacles – pull in water &
capture food
- Tentacles contain stinging cells called cnidocytes
- Each cnidocyte has a coiled stinger called a
nematocyst that discharges to either trap or
immobilize prey (able to differentiate between
animate and inanimate object, doesn’t just fire at
anything). The nematocyst injects venom.
- Tentacles guide paralyzed food into the mouth
- Nutrients are distributed via diffusion, waste is
expelled through mouth
loaded
Discharged 4
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Nervous System
- the mesoglea contains a
nerve net: a diffuse
network of nerve cells and
their extensions
- Mostly used for
coordination contractions
in body
- Sensory cells connect
the epidermis and
gastrodermis
statocysts
Simple sensory
“organs” in the
medusa:
1. statocysts are
used for balance
2. ocelli (eye
spots) detect light
REPRODUCTION
Asexual Reproduction
Polyps produce
new polyps or
medusae by
budding
Colonial hydrozoan
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Sexual Reproduction
Mature medusa
release gametes into
the water
After fertilization, the
zygote becomes a
ciliated larvae that
swims around
Eventually, the larva
attaches to a hard
surface and grows into
a polyp
Classes:
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