Download Cnidarians (Phylum Cnidaria)

Cnidarians (Phylum Cnidaria)
LEQ: What are the two body forms
of Cnidarians?
Cnidarians
• Sometimes called coelenterata. Include the
sea anemones, jellyfishes, and corals. More
than 9,000 species are known, almost all of
which are marine.
Cnidarians
• First animals to have tissues. Tissues allowed for
locomotion, responding to stimuli, and engulfing
prey.
Body Plan:
• Display radial symmetry (look the same from all
sides and have neither a head nor a front or
back).
Cnidarians
• Can occur as one of two basic forms: a polyp
(saclike attached stage) or a medusa (which is
like an upside-down bowl adapted for
swimming).
Medusa Stage
Polyp stage
Cnidarians
• The life history of some
cnidarians include both polyp
and medusa stages (jellyfish,
for example). Others spend
their entire lives as one of the
two.
Cnidarians
• The polyp and medusa share a similar body
plan. Both have a centrally located mouth
surrounded by tentacles, which are used to
capture and handle food.
Cnidarians
Feeding and Digestion
• The mouth opens into a gut where food enters
and digestion begins. The cnidarian’s gut does
not end with an anus. Their gut is usually
termed the gastrovascular cavity.
Cnidarians
• Practically all cnidarians are
predators, and must capture
their prey. Their tentacles are
lined with cells containing small,
fluid-filled capsules containing a
thread able to be quickly
ejected. The thread can be
sticky or armed with spines or
consist of a long tube that wraps
around parts of the prey.
Cnidarians
• The capsules (cells) are called cnidocytes.
These cells contain organelles called
nematocyst. Some nematocysts contain
venom.
Cnidarians
• Two layers of cells form the body wall of cnidarians:
the endoderm and ectoderm. A jellylike material can
be found in between the two cell layers. This
substance is composed largely of water and is called
mesoglea. In the medusa forms, the mesoglea makes
up most of the body wall.
Cnidarians
• The ectoderm cells contain contractile fibers.
Locomotion is accomplished by contraction of
these fibers. However, for the medusa
(swimming) forms, the strength of these
contractions is not great enough to overcome
water currents. Thus, medusae drift.
Cnidarians
• The endoderm cells line the gastrovascular
cavity. These cells release digestive enzymes
into the cavity to begin the digestion process.
When the food is partially digested, it is
engulfed by the endoderm cells, where
digestion is completed within food vacuoles.
Thus, digestion is both extracellular and
intracellular.
Cnidarians
Nervous and Respiratory Systems
• Though cnidarians lack a brain or true
nerves, they do have specialized nerve
cells. These cells interconnect to form
a nerve net that transmits impulses in
all directions. This simple nervous
system can produce some relatively
sophisticated behaviors. For instance,
some corals can recognize and kill
“imposters” in the colony.
Cnidarians
• Cnidarians have no
respiratory system or
circulatory system. They
obtain their oxygen and
excrete their wastes by
diffusion.
Cnidarians
• Classes of cnidarians include:
– The hydrozoans (class Hydrozoa):
hydroids (feathery or bushy colonies
of tiny polyps) and siphonophores
(hydrozoans that form drifting
colonies, such as the Portuguese
man-of-war)
– The scyphozoans (class Scyphozoa):
jellyfishes
– The anthozoans (class Anthozoa): sea
anemones and hard and soft corals.