Chapter 3: Phylum Cnidaria

Chapter 3: Phylum Cnidaria
Classes
Hydrozoa:
Scyphozoa:
Anthozoa:
Hydra, Obelia, Gonionemius, Physalia
Aurelia
Metridium
Class Hydrozoa
Hydra
Living Hydra is attached to the substrate by a basal disc. The stalk, or
stem, of the Hydra is called the column, or peduncle. It begins at the basal disc
and extends to the tentacles. A cone-shaped section rests on the peduncle. The
tentacles extend from the base of this cone. The cone is called the hypostome,
and is at its apex is the mouth. As you observe the peduncle you will notice
that the middle of it is lighter than the periphery. This is due to the presence of
a cylindrical cavity, which occupies the interior of Hydra. The cavity is called
the gastrovascular cavity; the only entry or exit into the gastrovascular cavity
is the mouth. Cnidocytes, or stinging cells, are the distinctive characteristic of
Phylum Cnidaria
Histology
Take a slide that shows a longitudinal and cross-section of Hydra and
study the histology. A cross section of the body should show two layers of cells.
The external layer is the epidermis. It is one cell layer thick, and most of the
cells are epitheliomuscular cells and appear rectangular. They have a nucleus
bearing a conspicuous nucleolus. At the base of these cells are smaller,
triangular cells with smaller nuclei. These are the interstitial cells. The other
prominent cells in the epidermis are cnidocytes.
The basal disc epidermis is basically of tall glandular cells. Their
glandular nature can be recognized by the numerous eosinophilic (red)
granules in their cytoplasm. Sensory cells are also present in the epidermis, but
are not demonstrated well with hematoxylin and eosin. The surface of the
epidermis is covered by a thin cuticle, which is very closely applied to the cell
membranes. The basal portion of the cells rest on the mesoglea. In Hydra, it is
very thin, appearing as no more than a bounding line. It is non-cellular. On the
interior of the mesoglea is a second layer of cells, the gastrodermis. The two
prominent types of cells in the gastrodermis are the digestive cells and the
glandular cells. Both are large cells. The glandular cells are recognizable by
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numerous eosinophilic globules, which are somewhat larger than those
encountered in the basal disc epidermis. The digestive cells do not have these
globules, but may have vacuoles, which are not stained. Both cells have a small
number of flagella projecting into the lumen (gastrovascular cavity or
coelenteron) of the organism. The flagella have been altered too much to be
visible in the preparations you have. Interstitial cells and sensory cells are
also present. Compare the external surface contour of the epidermis with the
internal surface contour of the gastrodermis.
Figure 2.1. Hydra cross-sections (A. female and B. male).
Figure 2.2. Hydra whole mounts (A. male and B. female).
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Reproduction
1.
2.
Asexual by budding: Look at slides of total mounts with buds and
longitudinal sections. After these buds attain a certain size, they
develop a mouth, hypostome and tentacles, then drop off and form
a basal disc.
Sexual: Under certain stimuli the epidermal cells of Hydra develop
regional thickenings, or gonads (testis or ovary). These appear as
bumps on the side of the peduncle. The testis (Fig. 2.2A) tends to
develop closer to the tentacles, while the ovary (Fig. 2.2B)
develops more basally. Look at the slides of the cross sections of an
ovary and testis. The testis results from interstitual cells
undergoing meiosis and producing sperm. These germ cells and
their precursors distinguish the testis as a structure made up of
many very small cells. The ovary, by contrast, has the large ova,
which are few in number, and adjacent somewhat smaller nurse
cells, which are numerous. Distinguish between the two organs
and their parts.
Hydroid Stage
The stage you have been studying is called the hydroid, or polyp stage.
This is one of the two stages present in the life cycle of most cnidarians. The
other stage is the medusoid stage (jellyfish). The polyp stage is found in all
three classes of the phylum, but is very small in the class Scyphozoa. Many
polyps, unlike Hydra and Metridium, are colonial. A common marine
Hydrozoan colony is Obelia. Look at a slide of the Obelia hydroid (Fig. 2.3).
There are two types of zooids (functional polyps) in a colony. The hydranth is
the feeding polyp, and is shaped much like the Hydra possessing tentacles. The
other zooid is reproductive, and is called the gonangium. On its stalk there are
many red rosette structures. These are medusa buds , which are produced
asexually and will eventually leave the colony and become free-swimming
jellyfish.
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Figure 2.3. Polyp form of an Obelia colony.
Another colony form is Physalia (Portuguese man-of-war). It has an air
bladder (pneumatophore) which functions for buoyancy. The long tentacles
hanging from the base of the bladder are supplied with nematocysts. They are
called dactylozooids and have the function of paralyzing prey. Immediately
under the bladder are gastrozooids, which digest the prey. They are
numerous. Gonophores (reproductive zooids) and jelly zooids are also
present. Physalia is, in reality, a mixture of a hydroid and medusoid generation
with some zooids, polypoid, and other medusoids. Observe the specimens on
demonstration.
Medusoid Stage
The medusoid, or jellyfish, stage is topologically identical with the
hydroid stage. The medusa differs from the hydroid in that it has more
tentacles, is free swimming, and has a much thicker mesoglea. Look at the slide
of the Obelia medusoid. This is a very small medusoid but it shows the convex
surface (exumbrella) and subumbrella (concave) surface. The projection
from the middle of the subumbrella surface is called the manubrium, and like
the hypostome of the hydroid contains the mouth at the apex. Frequently,
fixation of the specimen causes it to invert, making the manubrium project
atypically from a convex surface like a handle. In an oral or aboral view, four
canals can be seen radiating out from the center of the coelenteron. These are
the radial canals, which communicate with the central gastrovascular cavity
and a ring canal at the base of the tentacles. Single spherical objects are
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associated with each radial canal. These are the gonads. The histology will not
be drastically different from that of Hydra with the exception of the thick
mesoglea.
Obelia is too small for detailed study of one characteristic of the medusa
of the class Hydrozoa, and instead the jellyfish Gonionemus (Fig. 2.4) is used.
Gonionemus is structurally like the Obelia medusa, except that the broken
ribbon of gonads associated with them obscures the radial canals. Also, in
Gonionemus, one can recognize a bend in the tentacles near the end. This is an
adhesive pad. Projecting in from the inner edge of the ring canal region is a
shelf-like, membranous, structure called the velum. This is found only in
Hydrozoan jellyfish. Hydrozoan jellyfish are usually less than an inch in
diameter.
Figure 2.4. Scyphozoan jellyfish Gonionemus.
Class Scyphozoa
Scyphozoan Jellyfish
Anthozoans have no jellyfish stage. The large jellyfish, therefore, belong
to the class Scyphozoa. They do not have a velum, contain their gonads
internally, and possess sensory structures around the bell margin. Aurelia is an
example of this class. Size has been mentioned is one way to distinguish
medusae of this class (Fig. 2.5). The tentacles are short, and are arranged
around the periphery, as in other medusae. The manubrium is very short, and
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perhaps absent. From the mouth region there extend four long lobes called
oral lobes, or oral arms. They reach to the bell margin. The gastrovascular
cavity inside the mouth opens into it four large cavities, which when viewed
together, resemble a four leaf clover. From these cavities, called gastric
pouches, and between the gastric pouches radial canals go to the bell margin.
Thus, there are many radial canals, not just four, as with the Hydrozoan
medusa. On the oral surface of the gastric pouches are dark staining,
horseshoe-shaped, gonads. Also on the oral surface of the gastric pouches are
small projections called gastric filaments, which contain nematocysts. Around
the bell margin are periodic indentations. Lobes called lappets flank them.
Between each lappet are structures called rhopalia, which house a statocyst,
or organ of balance, a photoreceptor, and a chemoreceptor.
Figure 2.5. Scyphozoan medusa, Aurelia.
Class Anthozoa
Metridium senile (Sea anemone)
This organism is also a hydroid. However, it is in the class Anthozoa.
Look at a mature specimen on demonstration. Then, look at a cross-section. In
a cross-section, two distinctions are capable of being made with similar crosssections of Hydra and these serve to separate the two classes. Metridium has
the gastrovascular cavity divided into chambers by partitions called septa (Fig.
2.6). Some of the separations are complete, and are called primary septa. Or,
the partitions may extend only a short distance into the cavity, and are
secondary and tertiary septa. Secondly, the mesoglea is greater in extent
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than in Hydra. Locate the muscle bands in the larger septa. Locate the septa,
ostia, and acontia on the clay model.
Figure 2.6. A. Diagram of a sea anemone. B. Cross-section through an anemone.
Radial Symmetry
Throughout this exercise there has been no use of the orientation terms
used with organisms possessing bilateral symmetry. It is not meaningful to
speak of the dorsal or ventral surface of a hydroid or a medusoid. This is
because the organisms are radially symmetrical. Several planes can be
passed through the organism, which will divide it into mirror image halves.
Replacing the terms used in bilaterally symmetrical animals are the terms oral
surface, the surface on which the mouth is found, and aboral surface, the
surface opposite to the one on which the mouth is found. The main axis is the
oral-aboral axis. Movement away from this axis is peripheral. In this course
you will study three phyla that have radial symmetry.
Phylum Review
Phylum Cnidaria (means "nettle-bearing")
 sac-like body
 gastrovascular cavity
 lined by gastrodermis
 used for extracellular digestion
 single orifice for mouth and anus
 most have tentacles around oral end
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



nerve net; no central nervous system
polymorphism; polyps and medusoid stages
radial symmetry
nematocysts in cnidocytes
o Class Hydrozoa (means "water serpent animal")
 Hydra, Gonionemus, Obelia, Physalia
 solitary or colonial
 asexual polyps and sexual medusae--some may have either
polyp or medusa stage reduced or absent
 hydranths without mesentaries
 medusae with a velum
 aseptate gastrovascular cavity
 ectodermal gonads (when present)
o Class Scyphozoa (means "cup animal")
 Aurelia
 includes most large jellyfish
 solitary
 polyp stage reduced or absent
 bell-shaped medusae without velum
 microscopic septa divide gastrovascular cavity
 tetramerous radial symmetry
 endodermal gonads
 mesoglea enlarged
o Class Anthozoa (means "flower animal")
 Metridium (sea anemone)
 corals (both soft corals and hard corals)
 solitary or colonial
 only polyp forms; medusa stage absent
 gastrovascular cavity subdivided by mesentaries (septa)
bearing nematocysts
 usually either hexamerous or octamerous radial symmetry
 endodermal gonads
 mesoglea enlarged to form collenchyma (true mesoderm)
 Subclass Hexacorallia (means “divided into six”)
 sea anemones and hard corals
 hexamerous
 mesenteries in pairs
 Order Actiniaria
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o sea anemones
o well developed pharynx and siphonoglyph
 Order Scleratinia (means “hard”)
o hard corals
o calcareous exoskeletons
 Subclass Ceriantipatharia
 tube anemones and thorny corals
 mesenteries unpaired
 Subclass Octocorallia (means “divided into eight”)
 sea fans, soft corals, sea pansies
 octomerous symmetry
 soenia
Phylum Ctenophora
Ctenophores are structurally similar to Cnidarians in that they have two
embryonic germ layers and radial symmetry. They contain rows of comb-like
plates used for locomotion. Many are also luminescent and glow a bright green
color when disturbed. A jarred specimen is available for you to examine.
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Chapter 4: Phylum Platyhelminthes
Classes
Turbellaria:
Planaria, Dugesia dorotocephala
Trematoda:
Conspicuum icteridorum, Clonorchis (flukes)
Cestoda:
Taenia pisiformis (tapeworm)
Class Turbellaria
Planaria Whole Mount
Cilia, confined to the ventral side, provide locomotion for the organism.
Near the anterior end are 2 dorsal eyespots (ocelli) and lateral, triangular
auricles. The eyespots are collections of photoreceptors. The pigmented area
you see is merely a light absorbent area toward which the ends of the neurons
are pointed. As with most light receptors, the ends of the nerve cells are
pointed away from the incoming light. Are the ends that are stimulated
dendrites or axons? Review the neuron if you do not know. The auricles
contain chemoreceptors and thigmoreceptors. When planarians feed, the
protrusible pharynx projects out the mid-ventral mouth. Note the darkly
colored, highly branched intestinal caeca that can be observed through the
translucent body wall.
Prepared slides are available to you. On each slide is a whole mount of a
planarian, which has been fed a black pigment and a second planarian that has
not been fed the pigment. The slide is thickly mounted, and you should not try
to use the high power of the microscope. The organism, which has been fed
pigmented material, has its digestive system thoroughly stained. You can see
the pharynx and the three branches of the intestinal caeca. The branches
have smaller branches, and thus you have illustrated the dendritic (branched)
nature of the digestive system (Fig 4.1). Many of the organ units of
platyhelminthes have a dendritic pattern. Platyhelminthes lack an anus. The
digestive unit is thus referred to as an incomplete digestive system. Most
metazoans have a complete digestive system. Think of the problems that
accompany an incomplete digestive system.
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Figure 4.1. Planarian whole mounts.
Planaria Cross-Sections
Obtain a slide of three cross-sections through a planarian. Look at the three
cross-sections with a microscope. From what you have learned about the digestive
unit you should be able to distinguish the sections as anterior, mid-body and
posterior. Go to a higher power. You should be able to distinguish cilia on one
surface and thus locate the ventral side. There is a single layer of cells, which lines
the outside of the body. These cells bear cilia on the ventral side, and dark staining
rhabdites on both sides. The epidermis also produces glands, which have a reddish
secretion. These glands are larger at the edges of the organism, and there project
into the body.
Medial to the epidermis is a muscle layer. It has two parts: an external
circular layer, and an internal longitudinal layer. There are also transverse
muscle fibers that allow for more varied movement. Virtually all the interior
tissue is made of parenchyma cells, digestive cells, and muscle. The muscle
is present in thin strips oriented dorsoventrally, or transversely. The large
digestive cells surround the only cavities you see in cross-section. Nervous and
excretory tissue is present, but not easy to resolve. The remainder is thus
parenchymal tissue. Parenchyma introduces you to connective tissue. All of
the cells between the organs are parenchymal cells. They serve as a packing
tissue, but they are also important in regeneration. Parenchymal cells are
totipotent cells. That is, they have the capacity under appropriate stimulus to
develop into any other kind of cell in the body. Most metazoans lack cells with
this potential. Intriguing possibilities would exist if we could acquire that
potential in vertebrates. Regeneration in planarians is possible because of the
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totipotent nature of the cells and because of sufficient nutrition being available
while the digestive unit is regenerating.
Class Trematoda
Digestive system
Obtain a prepared slide of Conspicuum icteridorum, the gall bladder
fluke of the common grackle (Fig. 4.2). Label the parts as you find them in
looking through the dissecting microscope. The digestive system is much like
that of planarians, but with several modifications. First, the mouth is located at
the anterior end and is surrounded by a muscular oral sucker. Leading
posteriorly from the oral sucker is a tube like esophagus, which branches into
the two intestinal caeca that run almost to the posterior end of the organism.
The entire digestive unit resembles an inverted ‘Y’ with a short basal stem.
Around the esophagus and immediately posterior to the oral sucker is the
muscular spherical pharynx. The other non-reproductive system that is
somewhat visible is the excretory system.
Excretory system
At the posterior end is the excretory pore, which drains the large
excretory bladder. The excretory bladder will be visible as a central clear
area, since the walls of the organ are very thin. Two excretory ducts continue
forward, branch and eventually end as flame cells. Flame cells are only seen on
living specimens. Posterior to the bifurcation of the intestinal caeca is a
ventral sucker (acetabulum). This structure attaches the symbiont to the
wall of the gall bladder. It contains no opening into the interior; it is only a
holdfast organ.
Reproductive system
The male reproductive system begins with two dark staining round or
lobed testes just posterior to the acetabulum. From the testes, ducts (vasa
efferentia) go anteriorly to form a single vas deferens. The short vas deferens
ends in a large elongate cirrus sac, which opens via the male pore into the
genital atrium. Inside the cirrus sac is a basal enlargement, the seminal
vesicle, which narrows to form the cirrus. The cirrus is used to transfer sperm
to another worm by being projected out the male pore, genital atrium and
common genital pore and into the genital atrium of a second worm. Overview
of male reproductive flow:
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testes
vas efferens
pore
vas deferens seminal vesicle cirrus
genital atrium common genital pore
male genital
The female reproductive system begins with two unequally stained bodies
in the center of the organism. The darker of the two bodies is the ovary, and
the lighter the seminal receptacle. Both bodies are around a region called the
oötype. They have ducts connected to the oötype. The oötype cannot be seen
well, except in sections, but it is a very important crossroads and activity
center in the female reproductive system. We will discuss it in more detail in
lecture. The uterus arises from the oötype and the vitellarian ducts empty
into the oötype. The uterus obscures most of the organs. It is a coiled structure,
filled with many darkly staining eggs. After it leaves the oötype, the uterus goes
posteriorly and then anteriorly to open into the genital atrium through the
female pore. Lateral of the intestinal caeca are the vitellaria (yolk glands),
which drain into the vitellarian ducts. Overview of female reproductive flow:
seminal receptacle ® seminal receptacle duct
¯
ovary ® oviduct ® ootype ® uterus ® female genital pore ®
­
genital atrium ® common genital pore
vitellaria ® vitellarian ducts
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Figure 4.2. Diagram of a typical Conspicuum including reproductive and
digestive systems.
Class Cestoda
Obtain a prepared slide of Taenia pisiformis, one of the dog tapeworms
(Fig. 4.3). Do not use high power in observing this specimen. On the slide are
representative sections of one tapeworm. One unique feature is the absence of
a digestive system. Since the organisms are symbionts in the intestine of
vertebrates, they have come to depend on the host to provide the monomeric
nutrients.
The tapeworms have four basic regions. Anteriorly is the first region: the
scolex (head), which in this case contains four suckers, and an anterior
protrusion called a rostellum, which contains hooks. The hooks and suckers
provide a means of attachment.
Posterior to the enlarged scolex is the second basic region: the thin,
unsegmented neck. Though seemingly inconsequential except as a connecting
device it is the site of continuous asexual reproduction. The posterior end of
the neck region continues to produce new segments, proglottids, throughout
the life of the worm.
The initial proglottids produced by the neck are small and undifferentiated.
They are called immature proglottids, and are the third basic body region.
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Little internal structure is visible. However, as they grow and differentiate each
segment develops a reproductive system much like that of flukes. When the
reproductive system is visible the proglottids are called mature proglottids,
and are the fourth body region. Each mature segment is capable of mating. A
consequence of this is the production of large numbers of eggs, which are
frequently maintained in the proglottid. The segment becomes engorged as a
large egg sac and is then called a gravid proglottid. Laterally in each segment
is a longitudinal excretory duct. There are actually four of them and they
drain numerous flame cells. Longitudinal nerve cords parallel the excretory
ducts. The lateral genital pores frequently have the cirrus projecting from
them. Overview of male reproductive flow:
testes
vas efferens
pore
vas deferens seminal vesicle cirrus
genital atrium common genital pore
male genital
Overview of female reproductive flow:
vitellaria ® vitellarian ducts
¯
ovary ® oviduct ® ootype ® uterus
­
shell gland
Overview of flow of sperm into the worm:
common genital pore genital atrium female genital pore vagina
seminal receptacle seminal receptacle duct oötype
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Figure 4.3. The four areas of a typical cestode, including a detailed mature
proglottid.
Pseudocoelomates
Phylum Rotifera
Since rotifers are unable to swim against water currents, they are considered
to be planktonic. In contrast, organisms such as fish that can swim against
water currents are classified as nektonic. You should be able to discern a
hardened, segmented cuticle called a lorica (Fig. 4.4). Note the ciliated
corona, which is composed of 2 trochal disks and in life, whorls around and
gives the appearance of a wheel (hence the name of the phylum) connected to
the anterior end of the organism. The other characteristics of rotifers will be
best observed on whole mounts of rotifer slides. Check with your texts for
verification of individual features. Identify the pharynx, or mastax, that
contains two darkly staining jaws called trophi. In female rotifers, the ova and
yolk are produced in germovitellaria, which are combined ovaries and yolk
glands. The germovitellaria as well as intestine and excretory ducts empty
directly into a cloaca that, in turn, empties to the outside via the anus.
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Figure 4.4. Rotifer structures.
Phylum Acanthocephala
View the slides provided. Be able to identify the organism and know it is
a pseudocoelomate.
Phylum Review
Phylum Platyhelminthes (means "flat worms")
 many dendritic organ systems
 parenchymal tissue composed of totipotent cells (thus have capability of
complete regeneration of lost body parts)
 most primitive animals with excretory system
 acoelomate; lacks either eucoelom or pseudocoelom
 triploblastic (3 germ layers--ectoderm, mesoderm, endoderm)
o Class Turbellaria (means "stir like")
 planaria (Dugesia); brown planaria we use is Dugesia
dorotocephala
 free-living flatworms
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 integument (living, outer epidermis)
 ventral cilia
 incomplete digestive system since lacks anus; triradiate
intestine with numerous caeca or diverticula
o Class Trematoda (means "holes form")
 Conspicuum icteridorum, Clonorchis, Telorchis,
Cephalogonimus
 flukes: both monogenetic (1 host in life cycle) and digenetic
(2 hosts), sometimes trigenetic (3 hosts)
 tegument (nonliving, outer cuticle) lacking cilia
 incomplete digestive system; diradiate intestine
 endoparasitic
 lacks hooks; has oral sucker and ventral sucker
(acetabulum) for attachment to host
o Class Cestoda (means "girdle form")
 Taenia pisiformis
 tapeworms
 tegument lacking cilia
 no digestive system
 endoparasitic
 has anterior scolex with hooks for attachment to host
 body divided into series of proglottids
Phylum Rotifera (means "wheel bearing")
 very small, planktonic
 marine, freshwater, terrestrial; epizoic, parasitic

characteristic ciliated crown (corona) which looks like a wheel when beating
 shape correlates with mode of life
Phylum Acanthocephala (means "spiny head")
 proboscis with recurved spines
 lacks digestive system
 flattened body
 all are endoparasitic
 lacunar system for nutrient travel
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