Cnidarians
Jellyfish, Hydra,
Corals, Sea
Anemones,
Sea Fans and Sea
Pens
Cnidarians
 Soft-bodied
animals
 Stinging tentacles
 Sac Body Plan
 Tissue Level of
Organization
 Radial Symmetry
 Two basic body forms:
Medusa (bell- shaped) and
Polyp (flower-like)
Structure
 Mouth – both entry of
food and exit of wastes
 Tentacles – contain
spring-loaded
nematocysts to sting and
immobilize their prey
 Tentacles guide paralyzed
food into the mouth
 Gastrovascular cavity –
food is digested here
nematocysts
loaded
Discharged 
 Many cnidarians do not
have to eat due to
symbiotic relationships
with autotrophic protists
 The protists live in the
gastroderm of the animal
 Some cnidarians are so
dependent on the
relationship that they will
die if not kept in bright
sunlight
 Some cnidarians have
long, tube-shaped
branching
gastrovascular cavities
 They do not need to
send wastes back
through the mouth;
wastes simply diffuse
out through the cell
walls of the epidermis
 Cnidarians do not have
a BRAIN, but do have
nerve nets throughout
the body.
 The NERVE NET is
concentrated around
the mouth.
 Cnidarians
sense their
environments
using chemo
and touch
receptors in
the epidermal
layer
Sensory Receptors
Statocysts 
 Simple sensory
organs surround the
medusa bell
 1. statocysts are used
for balance
 2. Ocelli (eye spots)
detect light
 Cnidarians lack true muscle cells, but have cells that
change shape when stimulated by the nerve net –
therefore, they can move.
 The medusa can open and close like an
umbrella, allowing it to move by “jet
propulsion” when water is forced out of the
medusa
Asexual Reproduction
 Polyps produce new polyps or medusae by budding
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
Hydras- a Hydrozoan
 Spend most of life as
polyp
 Can move with a
somersaulting
movement
Portuguese Man-Of-War
 A type of hydrozoan that
grows in a colony
 One of the polyps
becomes a float to keep
the colony on the surface
 They may produce very
long tentacles – the
poison in the nematocysts
is very strong and causes
humans great pain
Jellyfish
 Most of life is as a
medusa
Box jelly 
 The sting of most jelly
fish are harmless to
humans, but the tiny,
Australian jellyfish can
kill you in 3-20
minutes.
Sea Anemones + Coral
 Only have the polyp
stage
 Most ecologically
important invertebrates
 Sea anemones are
solitary polyps
Sea anemones
Clownfish and Sea Anemone –
symbiotic relationship –tentacles protect clown fish from
predators, clownfish protects tentacles from being eaten
Corals
 Grow in shallow,
tropical water
 Produce skeletons of
CaCO3
 Most are colonial,
creating coral reefs
 Entire ecosystems
revolve around coral
reefs
 Coral reefs
also protect the
land from the
ocean swells
and waves
 Corals produce chemicals to prevent
being overgrown by other organisms.
Some of these chemicals have been
used as anti-cancer medications