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Phylum Cnidaria
(Coelenterata)
The “simplest” of the complex
animals . . .
Phylum Cnidaria
Cnidarians Include Hydras, jellyfish, sea anemones & corals
Radially symmetrical
Acoelomate
Two basic forms:
–Polyp: Cylindrical form which attach bases to substratum
–Medusa: Flattened, mouth down version of the polyp. Moves freely
Forms of Cnidarians
Types of Cnidarians
Sea
Anemone
Jellyfish
Hydras
Sea Coral
Simple Facts – Simple Creatures
Over 10,000 living species
– Both marine and
freshwater
Hydras in FW
Corals,
jellyfish, and
anemones in
marine
Radial symmetry
Tissue level of
organization
Body Plan
Basic Body Plan:
Has an outer epidermis
Has an inner
gastrodermis which
forms a gastrovascular
cavity
Has a middle "jelly filled"
layer called mesoglea
Body Plan
Two general body forms
exhibited
– POLYP
Sessile
Cylindrical body
Ring of
tentacles on oral
surface
– MEDUSA
Polyp Form
Medusa Form
Flattened,
mouth-down
version of
polyp
Free-swimming
Body Structure
Basic body plan of
ALL cnidarians
Sac with a central
digestive
compartment
Single opening
serving as both
mouth and anus
Ring of tentacles
on oral surface
Passive Predators!
All cnidarians are
carnivores
Tentacles capture and push
food into mouth
Tentacles are armed with
stinging cells
Cnidoblasts / cnidocytes
Contain stinging
capsules called
nematocysts
Nutrition
•They use 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
•There is no system of internal transport, gas exchange or
excretion; all these processes take place via diffusion
Cnidocytes
Cnidarians are predators which have tentacles
possessing batteries of special cells called Cnidocytes
Cnidocytes contain special stinging capsules called
nematocysts.
When triggered, these cells shoot out a discharge
thread which can entangle and/or poison it's intended
prey
How does the stinging cell work??
The tentacle is
stimulated
– Pressure on “trigger”
Nematocyst is
discharged
– Thread uncoils
– Entangles prey
Some species produce
toxins
– Injects toxin into prey,
paralyzing it!
Cnidocytes cover the length of tentacles
Caught ya’!! … (now what?!?)
Food is forced into the
cavity
Extracellular digestion
begins
Enzymes secreted into
cavity
Intracellular digestion
completes process
Partially digested food
engulfed by endoderm
cells
I’m Feelin’ it!
(Can Cnidarians feel their prey?)
First true nerve cells
in K. Anamalia
– Nerve net sends
impulses in all
directions
– Cells of epidermis
and gastrodermis
arranged into
contractile fibers
Do not have a
brain to receive
information!
Reproduction in Cnidarians
Varies among forms
Budding in Hydra
Hydras, anemones
and corals
– exist only in polyp
form
– Asexual
reproduction
(conditions good)
Budding
– Sexual
reproduction
(conditions
unfavorable)
Zygotes remain
dormant until
conditions
improve
Some colonial polyp
forms have a medusa
phase in their life
cycle
– Specialized
reproductive polyps
produce tiny medusas
by asexual budding
– Sexual reproduction
by medusas produce
ciliated larva
planula
– Planula settles and
develops into new
polyp
What about jellyfish?!?
Medusa and polyp
stages
– Separate sexes produce
gametes
– Fertilization occurs in
GVC of female
– Planula develops and is
free-swimming
– Planula settles, develops
into polyp
– Polyp produces new
medusas by budding
*Fig. 32-8, PG 690
The cnidarians may be a relatively small group
comprising the Animal Kingdom, but they play
a major role in any marine habitat.
3 Major classes
– Class Hydrozoa: Hydras
– Class Scyphozoa: Jellyfish
– Class Anthozoa: Anemones and corals
Class Hydrozoa: The Hydras
Polyp phase is
dominant
Generally microscopic
Generally freshwater
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 and mouth
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)
Class Hydrozoa cont.
Other Hydrozoans
Portuguese man-ofwar:
Single gas-filled float
with tentacles
Tentacles house the
polyps and modified
medusae of the colony
Class Scyphozoa: The Jellies
Generally referred to
as jellies
Medusa stage is
dominant
Come in a variety of
forms
Aggressive
predators, and can
be dangerous to
humans
Jelly
Portugese
Man-o-war
Class Scyphozoa
Jellyfish
• The
medusae are large and
contain massive amounts of
mesoglea
•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: Anemones & Corals
Appear like plants but are
animals.
Polyp form is dominant.
Corals build calcium
shells to protect
themselves
Anemone
– Have symbiotic relationship
with algae.
Can build extensive
masses which can form
land masses.
Coral Polyps
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
Coral Reefs
Clownfish living in
An anemone