Exercise 6: Poriferans and Cnidarians
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OBJECTIVES:
1. Describe how structures specific to poriferans and cnidarians help them survive and
reproduce in their environment.
2. List the fundamental characteristics of members of the phyla Porifera and Cnidaria.
3. Describe the body forms of cnidarians and describe reproduction of those species
alternating between polyps and medusa.
4. Compare the feeding methods of sponges, hydrozoans, scyphozoans, and anthozoans.
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PORIFERA
Like all animals, sponges (poriferans) are eukaryotic, multicellular, and ingestive-feeding
heterotrophs (derive their energy from organic molecules made by other organisms). Sponges
are the simplest of the major animal phyla, with most species living in the oceans. Sponges lack
tissues and organs and are typically
asymmetrical assemblages of cells - cells
perform a variety of functions (e.g.
nutritive, reproductive…) and appear
more independent of each other than the
cells of other animals. While sponges are
sessile, they are highly efficient filter/suspension-feeders (feed by straining
suspended matter and food particles from
the water, typically by passing water over
a specialized filtering structure).
The structure of sponges is simple
but effective for filtering gallons of
seawater daily (Fig. 1). Sponge walls
filter seawater and remove any food
particles. The outside of sponges is lined
by an epithelial layer of flat cells, which Figure 1: Morphology of a simple sponge.
is reinforced by spicules. Inside the
sponge is the spongocoel, a central cavity lined by flagellated cells call choanocytes. The
moving flagella of choanocytes draws water through pores within porocytes (epithelial cells) into
the spongocoel and across the collars of the choanocytes lined with microvilli to trap food
particles. Food is moved from the microvilli toward the base of the cell, where food is
incorporated into a food vacuole. The food vacuole is passed to an amoeboid cell, where
intracellular digestion occurs. Choanocytes produce a continuous flow of water into the sponge,
and eventually the filtered water exits through a large hole in the end of the sponge called the
osculum. A cross-section of a sponge (Fig. 2) reveals that the wall of a sponge is folded. Within
these folds there are alternating incurrent canals open to the outside of the sponge, through which
water enters the sponge, and flagellated canals opening into the spongocoel, which are lined with
choanocytes.
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Sponges reproduce asexually and
sexually.
Asexual reproduction includes
budding and the release of stress-resistant
aggregates of amoebocytes called gemmules.
In favorable conditions, amoebocytes in a
gemmule grow into a mature organism.
During sexual reproduction, choanocytes and
amoebocytes differentiate into gametes - eggs
remain in the mesenchyme (loose connective
tissue), but sperm are released into the water
and are captured by choanocytes or
amoebocytes of other sponges. The captured
Figure 2: Cross section of Granatia
sperm are transported to eggs, and
fertilization occurs. After a brief development, the embryo is expelled from the sponge. The
newly formed larvae may settle directly and transform into adult sponges, or may be planktonic
for a time. Most sponge species are hermaphroditic (i.e. have male and female reproductive
organs), but produce eggs and sperm at different times.
Sponges can provide protection for a number of small marine plants, which live in and
around their pore systems. Symbiotic relationships with bacteria and algae have also been
reported - the sponge provides support and protection and the symbiont provides the sponge with
food.
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Task 1 - INVESTIGATE SPONGES
Using Grantia as a model organism, you will now investigate the general structure of
sponges.
Procedure:
1. Examine the available sponges using your dissecting and compound light microscope.
2. Examine a prepared cross section of Grantia.
3. Examine a prepared slide of spicules, and locate spicules in the Grantia cross section.
Questions:
1. What features of a sponge clearly distinguish it as an animal?
2. Do sponges appear to have any organs?
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3. What is the importance of choanocytes for sponges? What organism(s) from previous
labs do choanocytes resemble?
4. What is the purpose of spicules in sponges? Would you expect them to be larger or more
abundant per unit area in larger sponges? Why?
5. What is the advantage of sponges having a folded wall?
6. What services do sponges provide for their respective ecosystems?
7. Fill in the appropriate information about sponges in your Taxa organization chart.
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INTRODUCTION TO CNIDARIA
Cnidaria includes three classes: Hydrozoa, Scyphozoa, and Anthozoa. Cnidarians are
mostly marine carnivores with radially symmetrical bodies. The body wall has two cellular
layers, and unlike sponges, cnidarians have true tissues (Fig. 3). The outer layer of cells
(ectoderm) and the inner layer
(endoderm)
surround
the
gastrovascular cavity; these
layers are separated by a
gelatinous, acellular mesoglea.
Amoeboid cells circulate in the
mesoglea.
Ectodermal cells
include
cnidocytes
and
muscular contractile cells.
Endodermal and glandular cells
secrete enzymes into the Figure 3: Basic morphology of a cnidarian polyp and
gastrovascular
cavity
for medusa. The body wall has two cellular layers: an
extracellular
digestion. ectodermis on the outside (dark blue) and an endodermis
Muscles and nerves occur in (orange) lining the gastrovascular cavity (light blue). A
their
simplest
forms
in gelatinous mesoglea (gray) separates the two true body
cnidarians, but cnidarians lack layers, and serves a role similar to a skeleton.
organs that centralize these
tissues to certain areas of the body plan.
Cnidarians have two basic body plans: polyps and medusa. Polyps are cylindrical
animals with upward-facing mouths surrounded by tentacles. Polyps are usually attached to the
substrate, and must wait for prey to come to them. Polyps may be solitary or colonial. In
contrast to polyps, medusa are
usually
free-floating
or
actively moving (swim by jet
propulsion: muscles squeeze
water out of the cavity inside
the bell), and umbrellashaped. Their mouths point
downward and are surrounded
by
hanging
tentacles.
Cnidarian
classes
are
distinguished primarily by the
relative dominance of the
polyp or medusa stage in the
life cycle. Some cnidarians
only occur as polyps or
medusa, but many alternate
Figure 4: Generalized cnidarian life cycle
between these two forms (Fig.
4). During the cnidarian life cycle, medusa produce and release eggs and sperm into the water
for fertilization. After fertilization, the zygote develops into a swimming mass of ciliated cells
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called a planula larva. The planula attaches to the substrate and develops into a polyp. The
polyp may reproduce asexually by budding other polyps or continue the sexual cycle by budding
immature medusa called ephyra, which develop into a mature medusa.
Most cindarians are carnivores, but some absorb dissolved organic chemicals, filter food
particles out of the water, or obtain nutrients from symbionts. Tentacles that surround the mouth
are used to capture prey, and are armed with stinging cells (cnidocysts) containing small, barbed
harpoon-like structures (nematocysts). Captured prey are pushed through the mouth into the
gastrovascular cavity, where extracellular digestion occurs, followed by phagocytosis of small
food particles and intracellular digestion (Fig. 5). In cnidarians, there is only one external
opening to the gastrovascular cavity (incomplete digestive tract), therefore food enters through
the same opening waste is eliminated from. As a result, cnidarians are restricted in their
consumptive and digestive processes.
Figure 5: Body plan and extracellular digestion in Hydra. The epidermis includes stinging cells
(cnidocytes), which can discharge a harpoon-like nematocyst.
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Task 2 - INVESTIGATE HYDROZOA
The polyp stage dominates the hydrozoan life cycle, although polyps and medusa occur
in most species. In colonial hydrozoans, the majority of polyps are specialized for feeding, and
some polyps are specialized for reproduction - lack tentacles and have buds from which the
medusoid stage is produced. Hydra are small, common hydrozoans, that live in shallow,
freshwater pools, and prey on small invertebrates. Hydra have no medusa stage - they are only
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found as polyps. Hydra are solitary and occasionally hang from the water’s surface with their
basal disk (terminal end of the body opposite of its tentacles) adhering to the surface of the
water. More often, however, they attach to a hard substrate with their
basal disk. To move, Hydra can detach themselves and somersault
along the substrate.
Other common hydrozoans include Obelia, Physalia, and
Gonionemus. Obelia typifies most hydrozoans because it has colonial
polyps and free swimming medusa (Fig. 6). These colonial polyps
appear plant-like and branch from a tube. Polyps of Obelia are
polymorphic (more than one adult form) - some are specialized feeding
polyps while others are reproductive polyps (Fig. 7). Four gonads lie in
the main body structure of Obelia, and when food is taken in through
the mouth, it moves into the main body structure and is distributed
Figure 6: Obelia
through a canal system. Each polyps in a colony is interconnected and
medusae
shares a continuous gastrovascular cavity. The stalk that each polyp
stems from is lined on the inside with cilia, which beat in order to create a current to move the
food and fluid throughout the colony.
Figure 7: Obelia hydroid colony with feeding and reproductive polyps.
Physalia is a floating colony of polymorphic
polyps. Some of the polyps form a gas-filled sac that
provides buoyancy and suspends other polyps that
comprise long tentacles used to capture prey that are
passed to feeding polyps. A sail on the float propels
Physalia about the sea, often in groups. Individuals
can become stranded on beaches, where their toxic
nematocysts can remain potent for weeks or even
months in moist conditions.
Gonionemus (Fig. 8 ) is a hydrozoan with a
large medusae and its tentacles have adhesive pads
that allow it to attached to submerged aquatic
vegetation to avoid being carried into unfavorable
environments by currents.
Figure 8: Medusa of hydrozoan
Gonionemus
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Procedure:
1. Obtain a living Hydra and observe it in a water-filled petri dish with your dissecting
microscope. If small, living crustaceans are available, place some near the tentacles of
the Hydra and observe the Hydra’s behavior.
2. Examine a prepared slide of a cross-section of a Hydra, and identify the ectoderm,
mesoglea, endoderm, and gastrovascular cavity.
3. Examine a prepared slide of Obelia in the medusa and polyp stages.
4. Examine all available preserved specimens of hydrozoans, including Physalia and
Gonionemus.
Questions:
1. How do Hydra respond to stimuli (touch, light, food)? What tissues are required for a
response of this manner?
2. What specialized cells are used by Hydra and other cnidarians to capture food? What
physical structure/mechanism is an appropriate comparison to these cells?
3. What structures of an Obelia polyp determine whether it serves in the acquisition of food
or in reproduction? Based on the prepared slide, which is more abundant in a polyp
colony?
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4. Compare the medusa of Obelia, Physalia, and Gonionemus.
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Task 3 - INVESTIGATE SCYPHOZOA
Scyphozoans are commonly called jellyfish, because the gelatinous medusa dominates
their life cycle, with the polyp reduced to a small larval stage. The mesoglea has amoeboid cells,
the gastrovascular cavity is divided into four radiating pouches, and the gastrodermis has
cnidocytes. Unlike hydrozoan jellyfish, scyphomedusae lack a velum (the circular membrane
beneath the umbrella that helps propel the hydromedusae), however they swim by contracting
and relaxing a ring of muscle fibers in the mesoglea around the rim of the dome. A nerve net
controls these contractions. Aurelia is a typical scyphozoan (Fig. 9), and its life cycle is typical
to cnidarians, except that its gametes are the only haploid stage of its entire life cycle. Aurelia
has four bright circular gonads that are under the stomach, and food travels through the muscular
body cavity while radial canals help disperse the food.
Figure 9: Aurelia life history.
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Procedure:
1. Examine prepared slides of Aurelia in its different life stages as well as any available
preserved specimens.
Questions:
1. What are some notable differences between similar life stages of hyrdozoans and
scyphozoans (e.g. size or structure of polyps, medusa…)?
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Task 4 - INVESTIGATE ANTHOZOA
Anthozoan polyps are solitary
or colonial, and there is no medusa
phase. Cnidarians in this class are
typically sessile, with the mouth
leading to a tubular pharynx and
gastrovascular cavity with separate
compartments partitioned by thin septa.
Anemones (Fig. 10 & 11) attach
themselves to the substrate with their
flat and sticky basal/pedal disk,
however this attachment is not
permanent, and anemones can slowly
slide on a film of mucus. When pieces
of the basal disk tear away from a
moving anemone the pieces can form a Figure 10: Sea anemone external body plan
new individual (asexual reproduction fragmentation). Corals are structurally similar to anemones, but corals are usually colonial and
much smaller. Most corals secrete a hard skeleton of calcium carbonate with many small cups
surrounding the polyps.
Procedure:
1. Obtain a preserved anemone and identify the key external and internal features (see Fig.
11), including the tentacles, mouth, pharynx, gastrovascular cavity, septum, and
basal/pedal disc.
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2. Examine the available pieces of dried coral and look for the small depressions where the
corals were located.
3. Based on the information and observations made of each class, fill in your Taxa
organization chart with information regarding Cnidaria.
Figure 11: Anatomy of an anemone. Septa increase the surface area of the gastrovascular cavity
to increase nutrient absorption during digestion
Questions:
1. Compare the anemone polyp to the hydrozoan and scyphozoan polyps observed earlier.
What features do anemones posses that increase their efficiency as an exclusively polyp
cnidarian class?
2. Do you think is it more advantageous for anthozoans to live solitarily or colonially?
Why?
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3. In the table below identify the major differences between each cnidarian class and note
the advantages/disadvantages of each.
Class
Hydrozoa
Distinguishing characteristics
Advantages/disadvantages
Scyphozoa
Anthozoa
EXERCISE SUMMARY
In this lab, you have examined the first two groups of animals. While the morphology,
physiology, behavior, and life history of Porifera and Cnidaria are relatively simple compared to
more complex animals like arthropods, their ability to adapt has made them extremely successful
aquatic animals. Several labs ago, you investigated protozoans like Paramecium that were
single-celled heterotrophs with all of the necessary structures for solitary existence. In this lab
you have examine two phyla of multicellular organisms with cell specialization - certain cells
perform specific tasks, and all of the cells must function harmoniously in order for the animal to
survive. Sponges have certain cells that provide support and protection, others that are
responsible for acquiring food, and some that serve as the main reproduction structures. While
these cells work together, sponges lack true tissues that cnidarians posses, which increases their
overall efficiency - cells are grouped together based on function and the overall structure of the
organism is dictated by maximizing biological success. While cnidarians do possess tissues,
they do not have true organs, and as you continue through the rest of the semester, you will
examine more complex animals with organs designed for specific purposes, as well as organ
systems that regulate specific characteristics of an animal’s life. Make sure you note the
important structures (and the functions of these structures) that distinguish each phyla in the
animal kingdom and how each makes it more or less complex/advanced than other phyla.
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