Marine Invertebrate Zoology
Phylum Cnidaria
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 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.
Classification
The phylum is divided into three classes:
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)
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Laboratory Procedures
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 colonial
animals, Obelia exhibits polymorphism or morphological specialization of its members.
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 free-swimming medusa.


Observe a stained whole mount of an Obelia medusa and observe its structure.
Draw a picture of an Obelia medusa.
Obelia Lifecycle
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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 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.
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 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 Mexico
and South Atlantic. Both exhibit polymorphism and habits similar to Physalia.
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.

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 the ring of gastric filaments within
the four gastric pouches.

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.
Reproduction and Life Cycle
Mature Aurelia medusae release gametes from the gonads into the gastrovascular cavity. 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.
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Under appropriate environmental conditions, the scyphistoma transforms into a
strobila. The strobila develops and releases by transverse fission a series of saucershaped ephyra larvae, which bear marginal sense organs and other medusoid features.
The ephyra larvae gradually transform into adult jellyfish to complete the life cycle.

1.
2.
3.
Study the following demonstration materials and draw a scyphistoma.
Scyphistoma of Aurelia or other scyphozoan (preserved or microscope slide).
Planula and ephyra larvae of Aurelia (microscope slide).
Strobila stage of Aurelia (microscope slide).
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.

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 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.


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 free-swimming
planula larvae. After a period as planktonic larvae, the planula settles on some hard substrate and metamorphoses into an
anemone.
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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 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.

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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Name ________________________________
Review Questions
1. From which 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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