Download Cnidarian

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
Nutrition
•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.
•
Stinging Organelles
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
•
Planula larva
Reproduction
• 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
•
Solitary Hydras
•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
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)
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-ofwar:
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
Class Scyphozoa
Jellyfish
•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
Scyphozoan Life Cycle - Aurelia
•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.
Class Anthozoa cont.
•Corals obtain much
of their energy from
microscopic
photosynthetic green
algae (zooxanthellae)
or dinoflagellates that
live symbiotically
inside the cells of the
coral