phylum Porifera

Lab exercise 2: Basal Animal Lineages: Porifera / Cnidaria
General Zoology Laborarory . Matt Nelson
phylum Porifera
SYMMETRY
The flatworms represent some of the simplest animals
that possess bilateral symmetry. The term bilateral
symmetry refers to a shape that has only one axis
that divides the body into two halves that are mirror
images of one another. Organisms such as
cnidarians have radial symmetry, i.e. there are
several planes that divide the body into halves that
are mirror images of one another. These two types of
symmetry, radial and bilateral, are the most common
types found in the Animalia. However, there are
more specific terms used to describe certain types of
radial symmetry (such as pentaradial symmetry) and
we will discuss these throughout the semester as they
become relevant.
Most of the sponges possess no body symmetry, and
are considered to be asymmetrical, although there
are some sponges that border on radial symmetry.
However, this apparent radial symmetry generally
does not apply to the pore system and the interior
regions of the body. In the future, when we discuss
body symmetry, it will be apparent that sometimes we are referring to the entire body, and sometimes only the
external features of the body. In humans, for example, the outside of the body generally possesses bilateral
symmetry, but the internal organs are in some cases asymmetrical.
Organization
Sponges are among the simplest of animals in terms of organization of the body. While they do possess
differentiated cells, they do not possess true tissues, or organs, or organ systems. In many ways, the body of a
sponge is not that different from a colonial group of cells. The general body structure is simple. There is an
outer layer of cells that surround the body, and a layer of cells that lines the interior surfaces of the body.
Sandwiched between these two layers is a gelatinous layer called the mesohyl which may contain skeletal
elements (spicules), spongin fibers which act as a flexible skeleton, and amoeboid cells which move about the
mesohyl carrying out various functions. Although there are specialized cells in each layer, none of these layers
is considered to be a true tissue. The functionally different cell types found in sponges are the key factor in their
classification as multicellular organisms. Sponges possess three major cell types:
pinacocytes - These flattened cells cover the outer body of the sponge. In some sponges these cells
are contractile and may possess the ability to alter the general shape of the sponge. In the simplest
sponges, modified pinacocytes called “porocytes” form the pores that allow water to enter the
body. Contraction of porocytes changes the size of the pore, limiting the movement of water
through the sponge.
amoebocytes (archaeocytes) - These amoeboid cells migrate throughout the mesohyl and are
responsible for carrying out intracellular digestion and transporting the products of digestion
throughout the body. Amoebocytes may also be specialized for other jobs, such as the production
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General Zoology Laboratory. Matthew K Nelson (2011)
of spongin fibers or spicules. Like stem cells, they are able to develop into any of the other
specialized cell types found in the sponge. As such, they are important in the reproduction of some
freshwater sponges, which will be discussed later in this exercise.
choanocytes (shown here) - These are flagellated cells similar in form to choanoflagellate algae. They
possess a collar which consists of a ring or microvilli (tiny cytoplasmic
extensions) that surround the flagellum and are bound together by
plasma membrane. The term “choanocyte” means “collar-cell.”
Choanocytes allow sponges to feed. These may line one or more
internal chambers in the sponge. The flagella of choanocytes beat
back and forth creating the flow of water which brings in tiny food
particles which stick to the collar and are ingested through
phagocytosis.
Most of the diversity in sponges involves the relative complexity of the canal system which carries water through
the sponge for suspension feeding. The simplest sponges possess a central chamber called the spongocoel
which is lined with choanocytes. Water comes into the spongocoel through pores (ostia) in the body wall, and
exits the spongocoel through a larger opening referred to as the osculum. These sponges are considered to
be asconoid. Asconoid sponges are usually relatively small sponges, being somewhat constrained by the
efficiency of their canal system. Syconoid sponges possess ciliated radial canals contained within the body
wall. Water flows into the dermal pores, and then enters the radial canals via small openings called prosopyles.
At its interior end, the radial canal opens into the spongocoel. This opening is the apopyle. The largest sponges
are leuconoid. Leuconoid sponges are the most common and generally the largest sponges. In leuconoids,
there is no spongocoel per se. Dermal pores allow water to flow into incurrent canals which lead to flagellated
chambers, where suspension feeding occurs. Excurrent canals carry water from the flagellated chambers to
the osculum.
apopyle
radial canal
prosopyle
dermal pore
syconoid body wall
asconoid sponge
leuconoid sponge
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General Zoology Laboratory. Matthew K Nelson (2011)
Obtain a Scypha sp. (Grantia sp.) whole mount slide. Observe the specimen with the light microscope
under low power, then switch to high power. Draw and label the specimen.
Classification
The body forms of sponges reflected by the levels of complexity in canal systems do not seem to be indicative
of relatedness. There are three extant classes of sponges:
Class Calcarea - This group possesses calcareous spicules (CaCO3). The spicules are monaxon, triaxon,
or tetraxon (straight, three pointed, or four pointed). These generally smaller marine sponges are
most common in the tropics. The Calcarea include asconoid, syconoid and leuconoid forms.
Class Hexactinellida - This class is referred to as the “glass sponges” because of the characteristic
siliceous spicules (SiO2) of its members. The name “hexactinellida” refers to the six-pointed
(hexaxon) spicules of glass sponges. These are all marine and either syconoid or leuconoid.
Class Demospongiae - Most of the sponges are in this group. These are all leuconoid, but can be
marine or freshwater. May have siliceous spicules or
spongin, or both. Commercial bath sponges are
made from members of this group
Obtain a bath sponge specimen. This is the spongin matrix
from an “ex-sponge.” Examine the sample under a
dissecting scope.
Freshwater sponges possess a special type of asexual
reproduction that is used to survive freezing during overwintering.
These sponges produce internal buds called gemmules (right),
which consist of archaeocytes surrounded by tightly packed
spicules. Archaeocytes eventually emerge from the gemmule
and develop into new sponges.
Obtain a prepared slide with a gemmule specimen. Observe
under low and high power. Note the spicules if visible.
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General Zoology Laboratory. Matthew K Nelson (2011)
phylum Cnidaria
The term Cnidaria ([knide = nettle] + [aria = like]) reflects the specialized stinging cells which cnidarians use for
defense and prey capture. This group was once part of a larger group called the coelenterates,
encompassing the Cnidaria and the Ctenophora (comb jellies). Since the coelenterates were not
monophyletic, they were broken into two separate phyla.
The body of a cnidarian is radially symmetrical. As a result, terms such as dorsal and ventral have no meaning
for these organisms. Instead, relative anatomical positions are referred to with respect to the position of the
mouth. The end of the body that is the location of the mouth is called the oral end, and the opposite end is the
aboral.
Organization
Cnidarians possess incipient tissue-level organization. They are diploblastic, meaning that they possess two
embryonic tissue layers. These two layers
eventually develop into the two layers of the
body: the epidermis and the gastrodermis.
Between the epidermis and the
gastrodermis is a gelatinous non-cellular
layer referred to as the mesoglea. The
epidermis comprises the outer layer of the
body. Epidermal cells are contractile and
can be used to change the shape of the
body. Cnidocytes are also part of the
epidermis. Cnidocytes are stinging cells
unique to cnidarians, containing special
stinging organelles called nematocysts. The
gastrodermis lines the gastrovascular cavity,
a chamber connected to the mouth.
Cnidarians possess two body forms: polyp
and medusa. These two body forms are
found within the same species. Normally, the life cycle of cnidarians alternates between the polyp and
medusa stage. Generally, the main function of the polyp is feeding, while the medusa is mainly concerned
with sexual reproduction. The polyp is the sessile form. The aboral surface is usually attached to a substrate,
while the oral surface is directed upward to allow for efficient feeding. The medusa is motile, swimming with
the oral surface downward. In some groups, one or the other of these forms may be reduced or absent.
Anemones, for example, do not have a medusa stage.
Nematocysts
The nematocysts of cnidarians possess a special tactile trigger called a
cnidocil. When the cnidocil is engaged, the nematocyst is discharged,
extending a long thread that punctures the prey item, injecting it with
venom. The nematocysts of some cnidarians can deliver a serious sting to
even a large organism such as a nosy human.
Examine the demonstration slide of a discharged nematocyst.
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General Zoology Laboratory. Matthew K Nelson (2011)
Classification
There are four classes of cnidarians.
Class Hydrozoa - Freshwater or marine. Often colonial. Members usuallly possess both polyp and
medusa phases.
Class Scyphozoa - Most of the jellies. Generally, scyphozoans are medusae. The polyp is reduced in this
group and usually exists as a scyphistoma, that produces medusae through strobilation.
Class Cubozoa - Box jellies. polyp develops directly into medusa without strobilation.
Class Anthozoa - Corals (colonial) and Anemones (solitary). mutualistic associations with single celled
algae give these bright coloration.
Hydrozoa
Obelia (Order Hydroidea)
The hydrozoan Obelia sp. is an excellent example of a colonial polyp. You should be able to easily distinguish
feeding and reproductive polyps in the colony. Gastrozooids (hydranths) are feeding polyps that capture food
items with tentacles. Food particles are ingested through the mouth, and passed throughout the colony via a
common gastrovascular cavity. Gonozooids (gonangia) are reproductive polyps that produce internal buds
through the process of strobilation, releasing medusa into the surrounding water. You should be able to see
these chains of medusa buds inside each gonangium. The entire colony is surrounded by a transparent sheath
called the perisarc. This chitinous protective covering is produced by all members of the colony.
Obtain a whole mount Obelia colony prepared slide. Observe under the compound microscope. Draw
and label: gonangia, hydranths, gastrovascular cavity, tentacles, perisarc.
Obtain a whole mount Obelia medusa prepared slide. Observe under the compound microscope.
Observe the preserved Obelia colony in alcohol.
Hydra sp. (Order Hydroidea)
Hydra sp. are ubiquitous solitary freshwater hydrozoans. These feed mainly on small crustaceans such as
copepods and Daphnia that they capture with tentacles. When stimulated, nematocytes in their tentacles
discharge nematocysts, which inject their prey with venom, immobilizing and killing it. The tentacles then curl
inward, directing the prey into the mouth. Hydra have a very simple sac-like gastrovascular cavity, and can
only accommodate one or two prey items at a time. Waste must also be discharged through the mouth,
limiting foraging efficiency.
Hydra have no medusa form, and reproduce both sexually and asexually. Commonly, Hydra produce asexual
buds that develop into nutritionally independent organisms. This daughter polyp may remain attached for
some time, or break off of the parent. Hydra polyps generally attach to a substrate and extend tentacles to
feed on passing organisms, but they may also attach to fragments of debris suspended in the water.
The body of a Hydra comprises two layers of cells, each of which is just one cell thick. The epidermis is the outer
layer of the body, and the gastrodermis lines the gastrovascular cavity. The epidermis has contractile
epitheliomuscular cells that are contractile and allow the hydra to respond to stimuli by changing the overall
shape of the body. The body can lengthen, contract, twirl or bend to one side.
Obtain a sample of live Hydra and observe it under the dissecting scope. Note how it moves. After
observing the hydra by itself, obtain a sample of Daphnia, and add them to the dish containing your hydra.
The hydra should begin initiate feeding behavior almost immediately. The body should extend, and the
tentacles should begin slowly moving back and forth. Eventually, a Daphnia will blunder into the hydra and
be captured by the tentacles. Observe the hydra feeding.
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General Zoology Laboratory. Matthew K Nelson (2011)
Obtain a whole mount Hydra preserved slide. Observe using the compound microscope.
Portuguese man-o-war (Order
Siphonophora)
Like other siphonophores, the portuguese man-o-war (Physalia
sp.) is a colonial hydrozoan polyp that is often mistaken for a
medusa. The colony has feeding polyps called gastrozooids,
reproductive polyps called gonozooids, and stinging polyps
called dactylozooids. The colony is kept afloat by a large
modified polyp called a pneumatophore. The
pneumatophore is filled with gas, and serves as both a float
and a sail.
Observe the preserved Physalia.
Fire corals (Order Hydrocorallinae)
Fire corals are colonial hydrozoans that secrete a calcareous exoskeleton like true corals. Despite their
apparent outward similarity, the fire corals (unlike true corals) can deliver a painful sting to humans unwise
enough to touch them.
Scyphozoa (order Semostomae)
Most of the species that are recognized as “jelly fish” are in the class Scyphozoa. These medusa are dioecious
and reproduce sexually. For the most part, they are pelagic and planktonic, traveling mainly through the
action of ocean currents. They do swim, but only short distances, and with little directional control.
The largest scyphozoans can reach sizes of up to 40
meters in length. Despite their relatively large sizes,
jellyfish still only consist of two layers of body cells. The
outer layer of the body is the epidermis, and the
gastrovascular cavity is lined by the gastrodermis.
Between these two layers of tissue is a gelatinous
mesoglea layer which can be quite large and make up
most of the body mass.
The polyp phase is reduced in the Scyphozoa. Adult
medusae are gonochoristic, and produce either eggs
or sperm. Fertilization is external, and the fertilized egg
develops into a planula larva. The planula eventually
settles to the bottom and develops into the
scyphistoma (polyp). The scyphistoma may reproduce
asexually to form podocysts which develop into more
scyphistomae, or produces ephyra buds through the
process of strobilation. These ephyra develop into freeswimming adult medusae.
Adult medusae are oriented with their aboral surface
up, and their tentacles and mouth directed downward.
Some scyphozoans possess nematocysts which can
deliver a powerful sting to a careless swimmer or beach
goer. The venom from a jellyfish sting can result in
redness, swelling, extreme pain, cramping, and even
death.
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sperm
egg
adult medusa
planula
scyphistoma
young medusa
strobila
General Zoology Laboratory. Matthew K Nelson (2011)
gastric pouch
stomach
gonads
oral arm
Observe the preserved Aurelia specimen. note
the tetraradial symmetry which is evident in the
position of the gastric pouches. Draw and label:
gastric pouch, stomach, gonads, oral arms,
tentacles, mouth
Anthozoa
Anthozoans lack the medusa body form and exist
as either solitary or colonial polyps. The colonial
anthozoans are generally referred to as corals and
the solitary anthozoans are called anemones.
Corals (order Scleractinia)
examine the coral skeleton examples.
Corals secrete a calcareous exoskeleton which
forms the basis for the colony. Each individual
polyp in the colony would have been anchored to
one of the tiny pits that is evident across the
surface of the coral skeleton. A living coral colony
is often brightly colored due to the presence of zooxanthellae (mutualistic algae) and protein pigments
produced by the coral. This mutualistic association with single-celled algae supplements the nutritional
requirements of the colony and is essential for survival of the coral. When corals become stressed by
environmental factors, they discharge their zooxanthellae and “bleaching” occurs. Coral bleaching is a
symptom of an unhealthy reef. Discharge of zooxanthellae can help the colony to survive short-term stresses,
but if stressful conditions persist, can result in death of the colony.
Each polyp has a gastrovascular cavity that is branched to increase its surface area. The gastrovascular
cavities of polyps near each other are interconnected by gastrovascular canals allowing for exchange of
nutrients.
Anemones (order Actinaria)
examine the preserved specimen of the
anemone Metridium.
Anemones are usually larger solitary anthozoan
polyps. Anemones also possess zooxanthellae,
and as a result are often brightly colored just like
their smaller coral counterparts. However, they
lack the calcareous exoskeletons produced by
corals, and as such are fairly soft-bodied. The
contractile cells of the epidermis work in
opposition to the mesoglea sandwiched
between the epidermis and the gastrodermis,
functioning as a hydrostatic skeleton.
Unlike corals, some anemones can inflict a
painful sting in response to human contact. The
tentacles of anemones possess many cnidocytes
which discharge in response to tactile stimuli.
They use their tentacles to catch relatively large prey items such as fish and crustaceans.
The gastrovascular cavity of anemones is highly complex, with a large surface area for absorption of nutrients,
and production of enzymes. Just inside the mouth, the pharynx leads downward into the gastrovascular cavity,
which is divided into multiple chambers by mesenteries which project inward from the body wall. Despite their
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General Zoology Laboratory. Matthew K Nelson (2011)
large size, the body of the anemone is
still only two cell layers thick, but these
two cell layers are folded back on
themselves many times to create a
tough, sturdy body and a subdivided
gastrovascular cavity.
Anemones are generally sessile,
although they can glide slowly along a
substrate. They attach to a substrate
using their muscular mucus covered
pedal disc. The pedal disc is also
involved in asexual reproduction.
Lacerations of the pedal disc often
result in binary fission of the anemone.
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General Zoology Laboratory. Matthew K Nelson (2011)
NAME: ________________________ SECTION:______________
LAB EXERCISE 2
questions
porifera
1. Which of the body forms found in sponges is present in Scypha?
2. Why do some sponges feel “spongy”?
3. Explain how water passes through a sponge.
cnidaria
1. Characterize the two body forms found in cnidarians:
2. What is a nematocyst?
3. The portuguese man-o-war is often mistaken for a jellyfish. How are siphonophores different from jellies?
4. How are corals and anemones different?
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General Zoology Laboratory. Matthew K Nelson (2011)
Drawings
Obelia sp. colony
Kingdom: _____________
Phylum: ______________
Class: ________________
Order: ________________
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General Zoology Laboratory. Matthew K Nelson (2011)
Drawings
Aurelia sp.
Kingdom: _____________
Phylum: ______________
Class: ________________
Order: ________________
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General Zoology Laboratory. Matthew K Nelson (2011)