Phylum Cnidaria
Cnidarians - jellyfish, coral, hydra,
sea anemone
*Named after the stinging cells found on
their tentacles called CNIDOCYTES
Cnidarians
• Are soft bodied, carnivorous animals that have
stinging tentacles arranged in circles around
their mouths.
• The simplest animals to have body symmetry
and specialized tissues.
General Characteristics
About 9000 species
Simple nervous system
Diploblastic organization
Endoderm
Ectoderm
Most Cnidarians have radial symmetry
General Characteristics Cont’d• Either tubular/vase or bell/cup shaped
• Only animal with
cnidocytes
• Cnidocytes-specialized
stinging cells (on tentacle)
• All marine (aquatic)
Cnidarians have two body forms:
polyp (vase shaped)
medusa (cup shaped)
Body Plan
• Cnidarians’ bodies are
organized around the
gastrovascular cavity,
where digestion takes
place
• GV has a single opening
that serves as both
mouth and anus
Body Plan
• Cnidarians typically have a life cycle that
includes two different looking stages: a polyp
and a medusa.
– Polyp: a cylindrical body with arm like tentacles
– Medusa: a motile, bell-shaped body with the
mouth on the bottom.
2 Layers of Cells
–Diploblastic:
–Gastrodermis: lines GV cavity
–Epidermis: lines the outside of
the body (the skin)
Cnidarian Body Wall
Stinging Cells
• Have Cnidocytes
–Contain nematocysts: barbed
thread-like structures inside the
cnidocyte that shoots out and can
puncture and inject a toxin or
entangle prey
Cnidocyte
Feeding
• Gastrovascular cavity- a digestive
chamber with one opening
• Inner gastrodermis secretes digestive
juices into gastrovascular cavity which
digests food and circulates nutrients
Feeding Cont.
Nervous and Movement
•sensory receptors
•Both polyp and medusa
have a nerve net
•A loosely organized
network of nerve cells that
together allow cnidarians
to detect stimuli
•Muscles help in
directional movement
& capturing prey
Life Cycles
• Cnidarians have two body forms that
are representative of the sexual and
asexual life cycles that they undergo.
–Polyp
–Medusa
Polyp
–Usually attaches to a substrate
at the aboral end
–Oral end is surrounded by
feeding tentacles
Medusa
Sexual
Shaped like an upside-down
bowl
Tentacles surround the
mouth that is housed
underneath the bell
Respiration, Excretion and
circulation
• Following digestion, nutrients are
usually transported throughout
the body by diffusion.
• They respire and eliminate the
wastes of cellular metabolism by
diffusion through their body cells.
Reproduction
Both sexual and asexual
Sexual
Gametes released into water
Fertilization takes place to
create free-swimming planula
Asexual
By budding
Generalized Cnidarian Life Cycle
Examples
• Scyphozoa: Jellyfish
• Anthozoa: Sea anemones & Corals
• Hydrozoa: Hydra
• Cubozoa: Box Jellies
Class Hydrozoa
• Ex. Hydras
The Hydra is a freshwater cnidarian
What kind of symmetry does hydra have?
Is it a medusa or a polyp?
Video of hydra at
http://www.microscopyu.com/moviegallery/pondscum/hydra/
• Includes hydras, fire corals, and
Portuguese Man of War
• Display alternation of generations
–Polyp
–Medusa
Portuguese Man of War - cnidarian that floats in the
water and has long tentacles
Class Scyphozoa
• True Jellyfish
–Small polyp stage during life
cycle
Class Anthozoa
• Polyps that are flower-like
• Includes sea anemones, corals,
sea whips, and sea fans
Coral Reefs are made from the skeletons of
cnidarians
Coral Characteristics
• One of the most biologically
diverse places in the world!
Coral characteristics
• Types of Corals:
1. Hermatypic – reef forming corals,
have zooxanthallae, found in warm,
shallow waters
Coral Characteristics
•
Zooxanthallae (type of Protist) form a
symbiotic relationship with coral
• Zooxanthallae gives coral its color
– Coral polyps provide shelter
– Zooxanthallae
Coral Characteristics
•
Two main types of corals (con’t)
2. Ahermatypic – non-reef forming, grow
in deep water, are found in polar to
tropical waters
Coral Characteristics
Coral reproduction
Most are monoecious (hermaphroditic)
External fertilization- release sperm and
eggs into the water
Many reproduce by budding as well (forms
coral reefs)
Reef Structure
• Anatomy of a coral polyp
Reef Problems
• Disease
• Warming water temps – may be the cause of
coral “bleaching”
• Increasing sediments
• Human footprints
Multimedia
Sponge Images and Videos http://www.junglewalk.com/video/Sponge-movie.htm
Cnidarian Images and Videos http://www.junglewalk.com/video/Coelantrate-movie.htm
General Invertebrate Movies at National Geographic
--- really good video on the portuguese man of war (under
invertebrates)
Hydra Video at http://www.schooltube.com/video/55045/Hydra