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Phylum Cnidaria
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)
• In between these tissue layers is a
noncellular jelly-like material called
mesoglea
Cnidarian Body Plans
Polyp form
• Tubular body, with the mouth directed upward.
• Around the mouth are a whorl of feeding tentacles.
• Only have a small amount of mesoglea
• Sessile
Medusa form
• Bell-shaped or umbrella 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
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
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
Stinging Organelles
• 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
• 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
Reproduction
• One of the most amazing adaptations is the
ability of some 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
Planula larva
Cnidarian Taxonomy
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
Solitary Hydras
• Freshwater hydras are found in ponds and
streams occurring on the underside of
vegetation
• Most possess a pedal disc, 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
Class Hydrozoa cont.
Colonial Hydrozoans - e.g., Obelia
• 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)
Class Hydrozoa cont.
Life Cycle of Obelia
• Gonozooids release free swimming medusae
• Zygotes become planula larvae, which eventually settle to
become polyp colonies
• The medusae of
hydroids are smaller
than those of
jellyfishes (C.
Scyphozoa)
• Also, the margin of
the bell projects
inward forming a
shelf-like velum
Class Hydrozoa cont.
Other Hydrozoans
Portuguese man-of-war:
Single gas-filled float with
tentacles
Tentacles house the polyps
and modified medusae of
the colony
Class Scyphozoa
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
Scyphozoan Life Cycle - Aurelia
• Gametes develop in
gastrodermis of gastric
pouches; eggs and sperm are
shed through mouth
• Fertilized eggs develop into a
planula larva; settles on
substrate and develops into a
polyp - scyphistoma
• Scyphistoma produces a series
of polyps by budding - strobila
• The polyps undergo
differentiation and are released
from the strobila as free
swimming ephyra
• Ephyra matures into an adult
jellyfish
Class Anthozoa
• 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
Class Anthozoa cont.
• 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