Phylum Cnidaria

Prepared by:
MARK D. CASABUENA
MILE - Biology

INTRODUCTI
ON The name Cnidaria comes from the Greek word
"cnidos," which means stinging nettle. Casually
touching many cnidarians will make it clear how they
got their name when their nematocysts eject barbed
threads tipped with poison.
 Many thousands of cnidarian species live in the
world's oceans, from the tropics to the poles, from
the surface to the bottom. Some even burrow. A
smaller number of species are found in rivers and
fresh water lakes.
 The Cnidaria include over 9,000 species are mostly
marine, and are important in coral reef ecosystems.

 Cnidarians are incredibly diverse in form, as
evidenced by colonial siphonophores,
massive medusae and corals, feathery hydroids,
and box jellies with complex eyes.
Yet, these diverse animals are all armed with
stinging cells called nematocysts. Cnidarians are
united based on the presumption that their
nematocysts have been inherited from a single
common ancestor.
INTRODUCTI
ON

Radial symmetry or modified as biradial
symmetry
 Diploblastic, tissue-level organization
 Gelatinous mesoglea between the epidermal
and gastrodermal tissue layers
 Gastrovascular cavity
 Nervous system in the form of a nerve net
 Specialized cells, called cnidocysts, used in
defense, feeding and attachment.
Characteristics of Phylum
Cnidaria include:

Body Wall and Nematocysts
 Cnidarians possess diploblastic, tissue-
level organization (see figure 1).
 Cells organize into tissues that carry out
specific functions, and all cells are derived
from embryological layers.
 The ectoderm of the embryo gives rise to
an outer layer of the wall, called the
epidermis, and the inner layer of the body
wall, called the gastrodermis, is derived
from endoderm (see figure 2).

Diploblastic Body Plan
Figure 1. Diploblastic Body Plan. Diploblastic animals
have tissues derived from ectoderm. Between these layers
is a noncellular mesoglea.

Body Wall of a Cnidarian (Hydra)
Figure 2. Cnidarian are diploblastic (two tissue layers). The epidermis is derived
embryologically from ectoderm, and the gasrodermis is derived embryologically
from endoderm. Between these is mesoglea. Mesoglea is normally acellular in the
hydrozoa, but it contains wandering mesenchyme cells in members of the other
classes. In the hydrozoa, cnidocytes are present only in the epidermis. In members
of other classes, they are present in both the epidermis and endodermis.

 Cells of the epidermis and gastrodermis
differentiate into a number of cell types for
protection, food gathering, coordination,
movement, digestion, and absorption.
 Between the epidermis and gastrodermis is
a jellylike layer called mesoglea. Cells are
present in the middle layer of some
cnidarians, but they have their origin in
either the epirmis or the gastrodermis.

 One kind of cell is characteristic of the phylum.
Epidermal and/or gastrodermal cells called
cnidocytes produce structures called cnidae,
which are used for attachment, defense, and
feeding.
 A cnida is a fluid-filled, intracellular capsule
enclosing a coiled hollow tube (see figure 3). A
lidlike operculum caps the capsule at one end. The
cnidocyte usually has a modified cillium, called a
cnidocil.
Stimulation of the cnidocil forces open the
operculum, discharging the coiled tube.

Cnidocyte Structure and Nematocyst
Discharge
Figure 3. Cnidocyte Structure and Nematocyst
Discharge.
(a) A nematocyst develop in the capsule in the cnidocyte. The
capsule is capped at its outer margin by an operculum
that is discharge of the nematocyst. The trigger like
cnidocil is resposible for nematocyst discharge.
(b) A discharge nematocyst. When the cnidocil is stimulated
a rapid (osmotic0 influx of water causes the nematocyst to
evert, first near its base, and then progressively along the
tube from the base to tip. The tube revolves at enormous
speed as the nematocyst discharge. In nematocysts
armed with barbs, the advancing tip of the tube is aided in
its penetration of the prey as a barbs spring forward from
the interior of the tube and then flick backward along the
outside of the tube.

 Zoologists have described nearly 30 kinds of
cnidae. Nematocysts are a type of cnida used
in food gathering and defense that may
discharge a long tube armed with spines that
penetrates the prey. The spines has hollow tips
that deliver paralyzing toxins.
 Other cnidae contain unarmed tubes that wrap
around prey or a substrate. Still other cnidae
have sticky secretions that help the animal
anchor itself.

Nervous system and senses
 Cnidaria have no brains or even central
nervous system. Instead they have
decentralized nerve nets consisting
of sensory neurons that generate signals in
response to various types of stimulus, such
as odors, motor neurons that tell muscles to
contract, and "cobwebs" of intermediate
neurons to connect them.
 As well as forming the "signal cables",
intermediate neurons also form ganglia that
act as local coordination centers.

Nervous system and senses
 The cilia of the cnidocytes detect physical
contact. Nerves inform cnidocytes when odors
from prey or attackers are detected and when
neighbouring cnidocytes fire. Most of the
communications between nerve cells are
via chemical synapses, small gaps across which
chemicals flow.
 As this process is too slow to ensure that the
muscles round the rim of a medusa's bell contract
simultaneously in swimming the neurons which
control this communicate by much faster electrical
signals across gap junctions.

Alternation of Generations
 Most cnidarians possess two body
forms in their life histories (figure 4).
 The polyp usually asexual and sessile.
It attaches to substrate at the aboral end,
and has a cylindrical body called the
column, and a mouth surrounded by
food-gathering tentacles.

Alternation of Generations
 The medusa (pl., medusae) is dioecious and
free swimming. It is shaped like and inverted
bowl, and tentacles dangle from its margins.
 The mouth opening is centrally located facing
downward and swims by gentle pulsations of the
body wall.
 The mesoglea is more abundant in medusa
than in a polyp, giving the former jellylike
consistency. When a cnidarian life cycle involves
both polyp and medusa stages, the phrase
“alternation of generations” is often applied.

Figure 4. Generalized Cnidarian Life Cycle.
The figure shows alteration of medusa and polyp
body forms. Dioecious medusae produce gametes that
may be shed into the water for fertilization. Early in
development, a ciliated planula larva forms. After a
brief free swimming existence, the planula settles to a
substrate and forms a polyp. Budding of the polyp
produces additional polyps and medusa buds.
Medusae break free of the polyp and swim away. The
polyp or medusa stage of many species is either lost
or reduced, and asexual xor sexual stages have been
incorporated into one body form.

Reproduction
 Most Cnidarians are dioecious. Sperm and
eggs may be released into the gasdtrovascular
cavity or to the outside of the body. In some
instances, eggs are retained in the parent until
after fertilization.
 A blastula forms early in the development, and
migration of surface cells to the xinterior fills
the embryo with cells that will eventually form
the gastrodermis. The embryo elongates to
form a ciliated, free-swimming larva, called
planula.

Reproduction
 The planula attaches to a substrate,
interior cells split to form the
gastrovascular cavity, and a young polyp
develops (see figure 4).
Medusae nearly always form by budding
from the body wall of a polyp, and polyps
may form other polyps by budding.
 Buds may detach from the polyp, or they
may remain attach to the parent to
contribute to a colony of individuals.

Regeneration
 All cnidarians can regenerate, allowing
them to recover from injury and to
reproduce asexually. Medusae have limited
ability to regenerate, but polyps can do so
from small pieces or even collections of
separated cells. This enables corals to
recover even after apparently being
destroyed by predators.

CLASS HYDROZOA
 Hydrozoans are small relatively common
cnidarians. The vast majority are marine,
but this is the one cnidarian class with
freshwater representative.
 Most cnidarians have life cycles that
display alternation of generations;
however, in some the medusa stage is
lost, while in others, the polyp stage is
very small.

CLASS HYDROZOA
 Three features distinguish hydrozoans
from other cnidarians:
1. Nematocyst are only in the epidermis;
2. Gametes are epidermal and are release
at the outside of the body rather than
into the gastrovascular cavity; and
3. The mesoglea is largely acellular.

CLASS HYDROZOA
 Most hydrozoans have colonial polyps in
which individuals may be specialized for
feeding, producing medusa by budding, or
defending the colony.
 In Obelia, a common marine cnidarian, the
planula develops into a feeding polyp, called
a gastrozooid or hydranth.
 Hydra is a common freshwater hydrozoan
that hangs from the underside of the floating
plants in clean stream and ponds.

CLASS HYDROZOA
 Hydra lacks a medusa stage and reproduces both
asexually by budding from the side of the polyp
and sexually.
 Hydra are somewhat unusual hydrozoans because
sexual reproduction occurs in the polyp stage.
 Large oceanic hydrozoans belong to the order
Siphonophora. These colonies are associations of
numerous polypoid and medusoid individuals.
Some polyps, called dactylozooids, possess a
single long tentacles armed with cnidocyst for
capturing prey.

 Obelia is a genus in the class Hydrozoa, which consists
of mainly marine and some freshwater
animal species and have both
the polypand medusa stages in their life cycle. The
genus belongs to the phylum Cnidaria, which are all
aquatic and mainly marine organisms that are relatively
simple in structure.
 Obelia has a worldwide distribution except the high-
arctic and Antarctic seas. The medusa stage
of Obelia species are common in coastal and offshore
plankton around the world.Obelia are usually found no
deeper than 200 metres (660 ft) from the water's surface,
growing in intertidal rockpools and at the extreme low
water of spring tides.
Obelia

Figure 5. Obelia Structure and Life Cycle

Figure 5. Obelia Structure and Life Cycle
 Obelia alternates between polyp and medusa
stages. An entire polyp colony stands about 1
cm tall. A mature medusa is about 1 mm in
diameter, and the planula is about 0.2 mm long.
Unlike Obelia the majority of colonial
hydrozoans have medusae that remain attached
to the parental colony, and they release gametes
or larval stages through the gonozooid. The
medusae often degenerate and maybe little
more than gonodal specializations in the
gonozooid.

A B
A Hydrozoan Medusa. (A) Gonionemus medusa
(B) Structure od Gonionemus

 Tubularia is a genus of hydroids that appear
to be furry pink tufts or balls at the end of
long strings, thus causing them to be
sometimes be called "pink-mouthed" or
"pink-hearted" hydroids.
 Their average height is 40–60 mm and the
diameter of the polyp and tentacles is 10mm.
Tubularia indivisa and Tubularia larynx can
be difficult to distinguish and the two often
grow together. In T. larynx the stems branch
while in T. indivisa they are unbranched.
Tubelaria

Tubularia larynx Tubularia indivisa

Figure 6. Tubelaria Structure and Life Cycle

 During the summer time, sperm are released
into the water and attracted to female
reproductive structures by means of a chemical
substance. Internal fertilization occurs in the
female medusoids.
 The fertilized eggs develop into
actinula. These larvae develop directly into a
new polyp. Although the medusa are attached
to the polyp, the life cycle resembles that of
typical Cnidarian with the polyp reproducing
asexually and the medusa producing egg and
sperm.
Life Cycle of Tubelaria

Hydrozoan Species
Physalia physalisCraspedacusta sowerbyi Sertularia argentea

 Members of the class Scyphozoa are all
marine and are “true jellyfish” because the
dominant stage in their life history is the
medusa. Unlike hydrozoan medusae,
scyphozoan lack a velum.
 The mesoglea contains amoeboid
mesenchyme cells, cnidocytes occurs in the
gastrodermis as well as epidermis, and
gametes are gastrodermal in origin.
CLASS SCYPHOZOA

CLASS SCYPHOZOA
 Many Scyphozoans are harmless to
humans; others can delivers unpleasant
and even dangerous stings.
 For example, Mastigias quinquecirrha ,
the so-called stinging nettle, is a common
Atlantic scyphozoan whose populations
increase in the late summer and become
hazardous to swimmers is to avoid helmet-
shaped jellyfish with long tentacles and
fleshy lobes hanging from the oral suface.

 Aurelia is a common scyphozoan in both
Pacific and Atlantic coastal waters of North
America the margin of its medusa has a
fringe of short tentacles and is divided by
notches.
 The mouth of Aurelia leads to a stomach
with four gastric pouches, which contain
cnidocyte-laden gastric filaments. Radial
canals lead from gastric pouches to the
margin of the bell.
CLASS SCYPHOZOA

CLASS SCYPHOZOA
 In Aurelia, but not all scyphozoans, the
canal system is extensively branched and
leads to a ring canal around the margin of the
medusa. Gastrodermal cells of all
scyphozoans possess cilia to continuously
circulate seawater and partially digested food.
 Aurelia is a plankton feeder. At rest, it sinks
slowly the waters and trap the microscopic
animals in mucus on its epidermal surfaces.
Cilia carry this foods to the margin of medusa.

Figure 7.
Structure of
Scyphozoan
Medusa. (a) Internal
structure of Aurelia.
(b) A section through a
rhopalium of Aurelia.
Each rhoplaium consist
of two sensory
(olfactory) lappets, a
statocyst, and a
photoreceptor acalled
the ocellus.

Figure 8. Aurelia Structure and Life Cycle
Aurelia Life History. Aurelia is dioecious, and as with all scyphozoans, the
medusa (10cm) predominates in the organism’s life history. the planula (0.3 mm)
develops into a polyp called scyphistoma (4mm), which produces young medusa,
or ephyrae, by budding.

SCYPHOZOAN SPECIES
Cyanea capillata Mastigias quinquecirrha Pelagia benovici

 Class Cubozoa includes jellies that have a
box-shaped medusa: a bell that is square in
cross-section; hence, they are colloquially
known as "box jellyfish." These species may
achieve sizes of 15–25 cm.
 Cubozoans display overall morphological
and anatomical characteristics that are similar
to those of the scyphozoans.
 A prominent difference between the two
classes is the arrangement of tentacles. This is
the most venomous group of all the cnidarians .
CLASS CUBOZOA

CLASS CUBOZOA
The cubozoans contain muscular pads
called pedalia at the corners of the square
bell canopy, with one or more tentacles
attached to each pedalium.
 These animals are further classified into
orders based on the presence of single or
multiple tentacles per pedalium. In some
cases, the digestive system may extend into
the pedalia.

 Nematocysts may be arranged in a spiral
configuration along the tentacles; this
arrangement helps to effectively subdue and
capture prey. Cubozoans exist in a polypoid
form that develops from a planula larva.
 These polyps show limited mobility along
the substratum. As with scyphozoans, they
may bud to form more polyps to colonize
a habitat. Polyp forms then transform into the
medusoid forms.
CLASS CUBOZOA

Figure 9.
Structure of
Cubozoan
Medusa

Figure 10 : Composite image of the life cycle of Morbakka
virulent, based on laboratory and field surveys. inter-stage
durations are also shown.

Cubozoan Species
Morbakka virulent Chironex fleckeri Carukia barnesi

Chironex fleckeri, commonly known as sea wasp, is a
species of deadly venomous box jellyfish found in
coastal waters from northernAustralia and New
Guinea north to the Philippines and Vietnam. It has been
described as "the most lethal jellyfish in the world", with
at least 63 known deaths in Australia from 1884 to 1996.
 Notorious for its sting, C. fleckeri has tentacles up to
3 m (9.8 ft) long covered with millions
of cnidocytes which, on contact, release microscopic
darts delivering an extremely powerful venom.
Facts about Chironex fleckeri

Facts about Chironex fleckeri
 Being stung commonly results in excruciating pain, and if
the sting area is significant, an untreated victim may die in two
to five minutes. The amount of venom in one animal is said to
be enough to kill 60 adult humans (although most stings are
mild).
 C.fleckeri was named after North Queensland toxicologist
and radiologist Doctor Hugo Flecker."On January 20th 1955,
when a 5-year-old boy died after being stung in shallow water
at Cardwell, north Queensland, Flecker found three types of
jellyfish. One of which was an unidentified: a box-shaped
jellyfish with groups of tentacles arising from each corner.

CLASS ANTHOZOA
 Members of class Anthozoa are colonial or solitary, and lack
medusae. Their cnidocytes lack cnidocils. They includes sea
anemones, stony and soft corals. Anthozoans are all marines
and are found at all depths.
 Anthozoan polyps differ from hydrozoan polyps in three
respects:
1. the moutrh of the Anthozoan leads to a pharynx, which is
an invagination of the body wall that leads into the
gastrovascular cavity;
2. mesenteries (membranes) that bear cnidocytesand gonads
on their free edges divide the gastrovascular cavity into
sections;
3. the mesoglea contains amoeboid mesenchyme cells.

CLASS ANTHOZOA
 Externally, anthozoans appear to show
perfect radial symmetry. Internally, the
mesenteries and other structures convey
biradial symmetry to members of this class.
 Sea anemones are solitary, frequently large,
and colorful. Some attached to a solid
substrates, and some live in symbiotic
relationships.
 The polyp attaches to its substrates by a
pedal disk and an oral disk contains the mouth
and hollow, oral tentacles.

At one or both ends of the slitlike mouth is a
siphonoglyph, which is ciliated tract that
moves water into the gastrovascular cavity to
maintain the hydrostatic skeleton.
 The Class Anthozoa also includes many
kinds of corals, including many reef-building
species. Reefs are formed by the calcareous
skeletons of many generations of coral polyps.
The polyps inhabit only the surface of the
reefs.
CLASS ANTHOZOA

These reefs are among the most productive
environments of the world, housing thousands
of species of fish and invertebrates, not to
mention plants and protists.
 Like some anemones, many corals are
inhabited by symbiotic algae called
zooxanthellae. These photosynthetic algae are
essential for those coral, which generally do
not live at depths to which light does not
penetrate.
CLASS ANTHOZOA

Figure 11.
Structure of
Anemone,
Meridium sp.

Figure 12.
A stony coral
polyp in its
calcium
carbonate
skeleton.

Figure 13.
Life cycle of Sea
Anemone

Figure 14.
Life cycle
of Corals

Orbicella annularis
Anthozoan Species

Ecology and Habitats
 Most polyps require solid substrata for
attachment, although a few burrow into soft
sediments, extending only their tentacular crowns
above the surface.
Polyps are abundant in shallow waters, but sea
anemones can also occur in the deepest parts of
the oceans.
 Medusae maintain a favored depth in the water and
are carried about by currents. Most hydromedusae
and scyphomedusae live in surface waters,
generally in bays and along coasts, but certain
species are abundant in the open ocean.

 Cnidarians generally occupy two major niches.
They may use their cnidocysts to trap prey items.
On the other hand, many cnidarians, anthozoans
in particular, depend on zooxanthellae, symbiotic
dinoflagellates within the tissues, to survive.
 These single-celled protists carry out
photosynthesis within the animal's tissues, and
pass on the carbon compounds they fix to their
hosts; corals, therefore, are photosynthetic
animals in a sense.

 Some cnidarians are nearly completely
dependent on zooxanthellae; others trap prey but
augment their diet with zooxanthellae.
 While not all corals are dependent on
symbionts — some live at great depths where
there is never light — colonial, reef-forming corals
depend on them; thus, reefs can only exist in
shallow water.
 Notice the white areas on this coral reef
exposed at low tide: this loss of symbionts, called
bleaching, is deadly to coral reefs.

Importance
 Prominent among organisms that foul water-
borne vessels are sedentary cnidarians,
especially hydroids. The muscles that make
scyphomedusae strong swimmers are dried for
human consumption in Asia. Sea anemones are
eaten in some areas of Asia and North America.
 Throughout the tropics where reefs are
accessible, coral skeletons are used as building
material, either in blocks or slaked to create
cement. Another use for cnidarian skeletons is
in jewelry.

 The pink colour known as “coral” is the hue of
the skeleton of a species of hydrocoral. Other
hydrocorals have purplish skeletons. Skeletons
vary in hue, and those considered most
desirable command a high price. The core of
some sea fans, sea whips, and black corals are
cut or bent into beads, bracelets, and cameos.
 All cnidarians have the potential to affect
human physiology owing to the toxicity of their
nematocysts.
Importance

Importance
 Most are not harmful to humans, but some can impart a
painful sting—such as Physalia, the Portuguese man-of-
war, and sea anemones of the genusActinodendron.
These, and even normally innocuous species, can be
deadly in a massive dose or to a sensitive person, but the
only cnidarians commonly fatal to humans are the
cubomedusae, or box jellyfish.
 Anaphylaxis (hypersensitivity due to prior exposure and
subsequent sensitization) was discovered with
experiments onPhysalia toxin. Extracts of many
cnidarians, mostly anthozoans, have heart-stimulant,
antitumour, and anti-inflammatory properties.

Phylum Cnidaria

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