Marine Invertebrate Zoology
Phylum Cnidaria
Classification
The phylum is divided into three classes:
Introduction
Cnidarians are the simplest animals with
definite tissues; the cnidarian body consists of two well-defined tissue layers
and a third intervening layer of
gelatinous material, the mesoglea, which
varies in structure among the three
classes of cnidarians. The outer
epidermis layer covers the external
surface of the body, and the inner
gastrodermis layer lines a single
internal body cavity. In the simplest
cnidarians, Class Hydrozoa, the
mesoglea is thin and gelatinous; in the
Class Scyphozoa, the mesoglea is
fibrous and contains amoeboid cells; and
in the Class Anthozoa, the mesoglea has
many amoeboid cells.
The cnidarians and the related Phylum
Ctenophora are especially noted for their
prominent radial symmetry. For this
reason, these two groups are commonly
referred to as the radiate phyla.
The name Cnidaria is derived from the
cnidoblasts, special cells, which
produce the nematocysts or “stinging
capsules” characteristic of this phylum.
The alternate phylum name,
Coelenterata, refers to the large coelenteron, or gastrovascular cavity, also
characteristic of the group. The related
Phylum Ctenophora derives its name,
meaning “comb-bearer,” from its
characteristic eight rows of comb plates,
or ctenes, which serve for locomotion.
The Cnidaria exhibits two basic body
forms: an attached polyp stage and a
free-swimming medusa stage. Many
species exhibit both a polyp stage and a
medusa stage, and their life cycles
involve an alternation of these two
body forms or “generations.” Two other
important distinguishing characteristics
of the phylum include tentacles around
the mouth and a diffuse nerve net,
which provides a modest degree of
nervous coordination.
CFCC
Class Hydrozoa (Hydroids and
Siphonophores)
Animals usually have both polyp and
medusa in the life cycle, medusae with a
velum, and gonads on radial canals of
medusae. Mesoglea is thin and
gelatinous; well-defined epidermis and
gastrodermis. Freshwater and marine
species. Examples: Obelia, Tubularia,
Halocordyle, Hydractinia, and Physalia
(Portuguese man-of-war).
Class Scyphozoa (True Jellyfish)
Mainly large marine jellyfish with
abundant mesoglea, polyp stage reduced,
no velum in medusae. Mesoglea
containing fibers and amoeboid cells. All
marine. Examples: Aurelia, Chrysaora
(sea nettle), and Stomolophus
Class Anthozoa (Sea Anemones and
Corals)
Solitary or colonial animals with polyp
stage only, medusa absent, pharynx or
gullet present, gastrovascular cavity
partitioned by septa. Mesoglea with
many amoeboid cells. All marine.
Examples: Aiptasia (sea anemone),
Astrangia (coral), Gorgonia (sea fan),
and Renilla (sea pansy)
Lab Exercises
A Colonial Polyp: Obelia
Class Hydrozoa
Obelia is a colonial marine cnidarian
which illustrates the complex life cycle
with alternating polyp and medusa
stages found in many cnidarians.
• Examine a stained whole mount of
the polyp or asexual stage of Obelia
and study its organization. Like many
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colonial animals, Obelia exhibits
polymorphism or morphological
specialization of its members.
polypoid and medusoid forms. The
familiar iridescent colonies of Physalia
are commonly found along the beaches
of Florida, the South Atlantic, the Gulf
of Mexico, and, sometimes, as far north
as Cape Cod.
Therefore you can distinguish two
different kinds of individuals in an
Obelia colony, feeding polyps or
gastrozooid and reproductive polyps or
gonozooid. The feeding polyps bear
tentacles armed with nematocysts, a
mouth, a hypostome, and a delicate outer
covering, the hydrotheca. Note that the
gonozooid have no mouth or tentacles.
How do they receive their nutrition?
The gastrozooid and gonozooid are
attached to a main stem, which consists
of a cylindrical tube of living tissue.
Alternation of Generations
The life cycle of Obelia illustrates the
alternation of generations’ characteristic
of the Phylum Cnidaria.
In the life cycle of Obelia, the polyp
generation produces medusa buds within
its gonozooid. The tiny, short-lived
medusae escape into the plankton and
produce either eggs or sperm. Fertilized
eggs develop into ciliated planula
larvae, which swim about in the sea for
a time and settle to transform into a new
polyp. Buds transformed by asexual
reproduction of the polyp do not detach,
thus forming a colony.
The sexual stage of Obelia is a freeswimming medusa
The most prominent feature of Physalia
is a gas-filled float above, which is a
sail-like crest. Physalia is transported by
winds and oceanic currents, and its
normal habitat is the open sea rather than
the sandy beach where it is most often
seen (and sometimes felt!) by bathers.
The sting from the nematocysts on the
tentacles of Physalia can be painful
when touched but is rarely dangerous,
except to highly sensitive individuals.
Below the float are suspended numerous
tentacles and other structures made up
of several kinds of modified polyps and
medusae. Thus, Physalia is an unusual
cnidarian since a single colony contains
both polypoid and medusoid individuals
of several types closely joined together,
in contrast to separate polypoid and
medusoid generations.
• Observe a stained whole mount of an
Obelia medusa and observe its
structure. Draw a picture of an Obelia
medusa.
Collection
Preserved specimens of Tubularia,
Physalia, and/or other hydrozoans.
•
A Portuguese Man-of-war: Physalia
Class Hydrozoa
The Portuguese man-of-war Physalia, is
a complex colonial hydrozoan (Class
Hydrozoa, Order Siphonophora)
exhibiting a high degree of
polymorphism.
A single colony may consist of as many
as 1,000 individuals and several types of
CFCC
Observe a preserved Physalia and
identify the float, the crest, and the
tentacles.
VeIelIa and Porpita are two related
colonial hydrozoans often seen on the
pacific coast and less often in the Gulf of
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Obelia Lifecycle
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Mexico and South Atlantic. Both exhibit
polymorphism and habits similar to
Physalia.
the ring of gastric filaments within the
four gastric pouches.

A Scyphozoan Jellyfish: Aurelia
Class Scyphozoa
Aurelia is a common, widely distributed
marine jellyfish. Large specimens may
reach 30 centimeters (12 inches) in
diameter. The polyp form, called a
scyphistoma, is small, sessile, and lives
attached to rocks and other submerged
objects in shallow coastal waters.
•
Reproduction and Life Cycle
Mature Aurelia medusae release gametes
from the gonads into the gastrovascular
cavity. How does this differ from the
hydrozoan medusae? The gametes exit
from the mouth and fertilization is
external.
The fertilized eggs or zygotes develop
into ciliated planula larvae, which may
be retained for a time on the oral arms of
the medusa and later, settle to the sea
bottom. There the larvae develop into a
small, trumpet-shaped polyp, called a
scyphistoma.
Under appropriate environmental
conditions, the scyphistoma transforms
into a strobila. The strobila develops
and releases by transverse fission a
series of saucer-shaped ephyra larvae,
which bear marginal sense organs and
other medusoid features. The ephyra
larvae gradually transform into adult
jellyfish to complete the life cycle.
Study a preserved specimen of
Aurelia to learn about the
organization of a scyphozoan
medusa. You will not need to
dissect the specimen, since the
transparent body readily shows most
important features. Handle the
specimen with care and return it for
study by another student.
Observe the four-part radial symmetry
and locate the four long oral arms
arising from the corners of the square
mouth. Along the arms find the many
short oral tentacles that help to capture
food (small planktonic animals), which
are then moved toward the mouth along
the ciliated groove on the oral side of
each arm. After passing through the
mouth, the food enters the gastrovascular
cavity. Internally the gastrovascular
cavity is divided into four gastric
pouches. A ring of gastric filaments
within each gastric pouch immobilizes
or kills any food organisms still active.
The gastric filaments bear many nematocysts.
Four horseshoe-shaped gonads surround
CFCC
Observe the complex branching
system of radial canals that distribute
food materials from the gastric
pockets to other parts of the bell, and
an outer circular canal around the
margin of the bell. Also, around the
margin of the bell, locate the eight
marginal sense organs. These
marginal organs are sensitive organs
of touch and balance.
• Study the demonstration materials
and draw a scyphistoma.
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1. Scyphistoma of Aurelia or other
scyphozoan (preserved or
microscope slide).
2. Planula and ephyra larvae of Aurelia
(microscope slide).
3. Strobila stage of Aurelia
(microscope slide).
attach to the central gullet; others (partial
or incomplete septa) bear thickened
septal filaments on their free inner
margins. The septal filaments extend
below the partial septa as thin, twisted,
threadlike acontia. The acontia bear
numerous nematocysts and act to subdue
living prey taken into the gastrovascular
cavity.
A Sea Anemone
Class Anthozoa
Sea anemones are typically sessile
cnidarians that attach to rocks, shells,
pilings, and other hard substrates in the
sea. Some species, however, burrow in
soft bottoms or are even free-swimming.
All nonetheless represent the polyp form
of the cnidarians; the medusa generation
is totally lacking in this class. The
anemones and other Anthozoa represent
the highest degree of specialization of
the cnidarian polyp. The basic features
of the anthozoan polyp are well
illustrated below.
• Observe the sea anemone under the
dissecting scope. Using a teasing
needle pierce the side of the anemone
and observe the acontia as they flow
out.
• Place a short piece of acontium on a
clean glass slide. Place a drop of
water on the acontium and cover with
a cover slip. Using the compound
microscope observe the acontium
under 40x and sketch the specimen as
it appears. Try to indicate which
nematocysts are discharged and
undischarged.
Nematocysts
Reproduction and Life Cycle
Reproduction in anthozoans and most
other anemones is both asexual and
sexual. Some anemones reproduce asexually by splitting longitudinally
(longitudinal fission), but the main
asexual means of reproduction is pedal
laceration. Bits of tissue from the pedal
disc are split from the anemone as the
animal moves along the substrate. These
tissue pieces later regenerate an entire
small anemone, literally in the footsteps
of its parent.
Sexual reproduction occurs seasonally
when gametes are released from the
gonads on the partial septa into the
gastrovascular cavity. The gametes are
released and are fertilized in the sea. The
fertilized eggs develop into freeswimming planula larvae. After a period
as planktonic larvae, the planula settles
on some hard substrate and
• Select a preserved sea anemone and
identify the mouth in the center of the
oral disc surrounded by many short
oral tentacles, and the basal disc,
which attaches to rocks or other hard
substrates.
Observe the feeding of living anemones
in an aquarium (if available) to better
understand the interaction of the
tentacles and ciliary currents on the oral
disc and in the gullet during the feeding
process.
The gastrovascular cavity is partially
divided into sections by thin vertical
walls of tissue called septa. Some septa
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metamorphoses into an anemone.
the stony corals are several reef-building
species, which are largely responsible
for the formation of many coral reefs in
warmer parts of the oceans, including
those in the Bahama Islands, off the
Florida Keys, and of the Great Barrier
Reef of Australia.
Some examples of stony corals are
Fungia, a solitary coral; Astrangia
danae, the Atlantic star coral; Oculina,
the eyed coral; and Diploria and
Meandrina, brain corals.
The octocorals exhibit strong eight-part
(octamerous) radial symmetry and have
an endoskeleton consisting of separate
microscopic pieces (spicules). A tough,
horny organic material is also present in
some species. This group is especially
prominent in tropical waters and includes the sea fans, sea whips; the sea
pens, the sea pansies, the organ pipe
coral, and the precious red coral
(Corallium) used for jewelry.
Corals
Class Anthozoa
Most anthozoans are corals, the largest
and best known of which are the stony
or scleractinian corals (Order
Zoantharia) and the octocorals (Subclass
Alcyonaria), which includes the soft and
horny corals. Corals, like all anthozoans,
have only a polyp form. There is no
medusa stage in the life cycle.
The polyp of a stony coral resembles a
sea anemone, although the individual
polyps are generally smaller than those
of anemones. The coral polyp sits in a
cup on the surface of a calcareous
exoskeleton secreted by the lower
portion of the column and epidermis.
Extending inward from the wall of the
cylindrical cup are several calcareous
septa, which extend from the sides and
base of the cup into folds of the basal
tissue of the polyp. These tissue-covered
septa partially subdivide the
gastrovascular cavity inside the polyp
into several chambers.
Most stony corals are colonial, and
adjacent polyps are connected by lateral
extensions of the body wall, which cover
the intervening stony skeleton of the
colony. These sheets of lateral tissue
also contribute to the formation of the
skeleton by their secretions and serve to
connect the gastrovascular cavities of
adjacent polyps.
Many growth forms occur among the
stony corals, and most species exhibit
characteristic skeletons. A few species
are solitary and occur as large individual
polyps.
Some stony corals contain symbiotic
algae (zooxanthellae), and many
interesting studies have been conducted
on the physiological and biochemical
interactions of these symbionts. Among
CFCC
• Study the demonstration materials
illustrating several types of coral and
draw several different types. Identify
each type of coral that you draw.
Examinations
1. Living sea anemones and/or corals
in a marine aquarium.
2. Representative preserved anemones.
3. Assortment of preserved corals and
dried coral skeletons.
*Material for this lab was taken from:
Lytle, C.F, and Woodsedalek, J.E. General
Zoology WM. C. Brown Publishers 1991
Hopkins, P.M. and Smith, D.G.
Introduction to Zoology Morton
Publishing 1997
Sumich, J.L. and Dudley, G. Laboratory
and field investigations in marine life
McGraw Hill 1998
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Review Questions
1. From what structures is the phylum Cnidaria derived?
2. In the Obelia (colonial hydroid) lifecycle which stage reproduces sexually and which
reproduces asexually?
3. What is the function of the strobila in the scyphozoan lifecycle?
4. Briefly, describe two methods that sea anemones use in asexual reproduction.
5. What is the function of acontia in the sea anemone?
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Cnidaria Laboratory Objectives
Hydrozoan polyp
Examine and sketch the prepared whole mount of a hydroid polyp. Label the different
hydroid polyp types and other structures discussed in the lab exercise.
Be sure to include sizes where appropriate.
Hydromedusa
Examine and sketch the prepared whole mount of a hydromedusa. Label structures,
which can be discerned. Use the hydromedusa sketch from lecture as your guide.
Be sure to include sizes where appropriate.
Order Siphonophora
Examine the preserved Portuguese Man-of-War.
Class Scyphozoa
Examine several different species of true jellies.
Sketch and identify the key structures using the finest specimen available.
Examine and sketch the prepared slide of a scyphozoan scyphostoma.
Examine and sketch the prepared slide of a scyphozoan ephyra.
Class Anthozoa
Obtain a piece of acontia (from a sea anemone) and prepare a wet mount using a
coverslip.
Observe and sketch both discharged and intact nematocysts.
Be sure to include sizes.
Coral examination
Examine the different types of corals, which are prepared for the practical exam.
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