UNIT 5 Notes #3 – Phylum CNIDARIA
A) Examples of Cnidarians
1.Hydras
-Class Hydrozoa
-Spend most of life as a polyp
- No medusa stage
2. Jellyfish
-Class Scyphozoa
-Jellyfish go through the
same life-cycle stages as hydrozoans.
-In Scyphozoans the medusa is large and longer
living, and the polyp stage is restricted to a tiny
larval stage.
-The nematocysts (stinging threads on tentacles)
of most jellyfish are harmless to humans, but a
few can cause painful stings.
3. Sea Anemones and Corals
-Class Anthozoa
-Anthozoans have only the polyp
stage in their life cycle.
B) Cnidarian Characteristics:
Body Plan:
- Radially symmetrical body with a single
opening.
- Opening will allow food to enter and wastes
to leave.
- Surrounding the mouth is a ring of
tentacles (usually contains stinging cells)
- Tentacles contain nematocysts; when
touched, the nematocyst thrusts out a springloaded poisonous thread into its prey,
paralyzing or trapping it.
- Separating the outer ectoderm from the
inner endoderm is a layer of mesoglea.
(Mesoglea offers support for the animal).
-The body has two alternate but similar body
forms.
a) POLYPS – Usually attached to some object
with the mouth and tentacles directed
upwards.
- Sessile (non-motile stuck to the bottom)
b) MEDUSA – Is free swimming with the
mouth and tentacles directed downwards.
- Locomotion is achieved by individual muscle-like
epidermal cells all working together to change
shape.
- Nematocysts fire on contact; after paralyzing its
prey, it pushes the food into its mouth with its
tentacles.
C) Various Body Systems:
1) DIGESTION: Food is pushed into the central cavity
where gland cells release enzymes in order to digest
food. Other cells of the endoderm will then absorb the
digested nutrients. Indigestible material will then be
eliminated out of the mouth.
2) CIRCULATION: There is no true circulatory
system. Food/nutrient particles will be passed through
the central cavity by body movements and by flagellated
cells in the endoderm. Thus the cavity is involved in
both digestion and circulation. This cavity is called the
gastrovascular cavity.
3) RESPIRATION and EXCRETION:
-Both take place by diffusion with the water that bathes
the tissues.
4) NERVOUS: There is a limited amount of nervous
and sensory tissue. This “nerve net” is used for coordination. Sensory cells are sensitive to touch and
various chemicals.
5) MUSCULAR: Lack muscle cells, but epidermal cells
can contract and change shape when stimulated.
6) REPRODUCTION:
Asexual- Many polyps will reproduce by budding, this
may form new polyps or medusae.
Sexual - Medusae reproduces sexually. Testes will
produce sperm which travel through the water to the
ovary which has produced eggs.
- The fertilized eggs are released out of the mouth to
develop into ciliated larvae (Planulae), this stage will
usually develop into a young polyp.
- Some Cnidarians (ex hydra, sea anemone) have
polyps that can also reproduce sexually.
D) Advantages of Motile and Non-Motile Stages/Forms
In Life Cycle:
a) If motile, organism can find new food; if non-motile,
you stay in one place and you can run out of food.
b) Advantage of being sessile (non-motile) is that they
can hide from predators and they don’t have to worry
about washing ashore.
c) Disadvantage of being motile is that you can easily
get carried away by the water currents.
E) Ecological Roles of Cnidarians:
- Form symbiotic relationships with other animals.
- Coral reefs are shelter for many marine animals.
They contain tunnels, caves, and deep channels in which
animals live. Coral reefs provide habitat for many fish
and food for many other animals that produce valuable
shells, pearls, and jewelry.
-Protect land from erosion by reducing wave action. If
destroyed, large amounts of shoreline maybe washed
away.
-Medical research of chemicals produced by many
Cnidarians. - New Antibiotics
- New Anticancer Chemicals