Download Cnidarian Observations

Cnidarian Observations
The animal Phylum Cnidaria is made up of animals with a primitive level of organization: they have
only two layers of cells, an outer, somewhat skin-like ectoderm and an inner digestive lining, endoderm
or gastroderm, and a layer of jellyish material loosely occupied with replacement cells in between. A
particular feature of this group is their cnidocytes, stinging cells containing poisoned harpoons that are
used for catching prey and defense. Many members of the Cnidarians live attached to the ocean floor such as corals and sea anemones - and some, such as jellyfish, swim weakly in the open water. In this lab,
we will look at a few examples from this group.
Prelab Questions:
1. What characteristics are unique to the phylum Cnidaria?
2. Using the terminology from our development unit, describe the body plan and development of
cnidarians?
3. What body systems do cnidarians have?
Cnidarians - Hydra
Hydras are common freshwater animals - you can find them during warm weather if you know what to
look for. They are one of the few successful fresh water Cnidarians, which makes them easy to keep in
the lab; however, they're not very typical Cnidarians. Hydras have a permanent polyp body form, which
in most Cnidarians is one of two alternating body forms, but Hydras lack the other, medusa stage, a
weak-swimming stage that would not do well in moving inland waters. Q: How are hydra different
from other cnidarians?
Lab Instructions. Get a pre-made slide of a hydra. Find a lens that will show the all of the hydra’s body
parts. Remember, the lowest objective lens has a power of 40x, the medium is 100x, and the highest is
400x.
1. On your WS, draw your Hydra and label the following structures: the BASE, a disk on the other end of
the BODY from the TENTACLES; in the mound where the tentacles meet the body, the MOUTH; the
DIGESTIVE CAVITY should show through the body as a shadow.
________X
2. Hydras have a variety of reproductive structures, both sexual (spermaries, slightly pointed bumps, eggs
or ovaries, broad low mounds, or embryos, like attached balls) and asexual (buds, like miniature Hydras
attached to the body). If your hydra has these structures, (Q2) describe them on your WS. If not, use the
textbook to observe the structures and describe them.
3. Obtain the pre-made slide of Physalia (Portuguese Man-of-War) nematocysts. Find and draw the cross
section of a tentacle. Label the egg-shaped trigger chambers, the cnidocyte (often there will be harpoons
visible inside), and see if you can identify the cnidocil. Q3: What is the function of the cnidocil?
_______X
4. Obtain the preserved specimen of the Portuguese Man-of-War (A7). On your WS draw and explain its
body plan. Include texture, size, color, ect.
Cnidarians - Obelia Hydroid Colony.
Polyp Form
Obelia is a small marine Cnidarian that lives in colonies, with several animals connected by stalks or
stems. There are two basic forms in the colony members: feeding and defending HYDRANTH polyps,
with spindly TENTACLES around a hidden mouth; vase-shaped GONANGIUM members give rise
asexually to the medusa stage of the Obelia, which are visible in many gonangia as small, disklike
MEDUSA BUDS (some gonangia may be empty, having released their medusae).
Q4: What are the two types of members found in the Cnidarian colonies? What are their roles?
5. On the microscope slide, find an area with both hydranths and full gonangia - the best area is toward
the middle of the colony. Draw enough of the colony to include all of the structures, and label those
structures.
________X
5. Obtain the preserved specimen of the Obelia Hydroid (A5). On your WS draw and explain its body
plan. Include texture, size, color, ect.
Cnidarians - Obelia Medusa Stage.
Get an Obelia medusa slide. You want a specimen that looks round. This is a medusa, tiny jellyfish .
Medusae reproduce sexually, producing a planula larva that will settle to the bottom and, if conditions are
favorable, become a new polyp colony. Draw and label a medusa - it is thick enough that it may not be in
focus all at once, and some structures may be setting on top of others. A medusa is like an upside-down
version of a polyp - the TENTACLES point down, attached to the edge of the bell. The MOUTH is also
underneath, leading into a MANUBRIUM chamber that connects to the central STOMACH. A series of
canals help distribute materials - four RADIAL CANALS connect the stomach to the RING CANAL
around the edge of the bell. Each radial canal travels past a GONAD. Each medusa is male or female, but
identifying which is which is very difficult.
Q5: Explain how the medusae reproduce?
_____X
Cnidarians - Aurelia Planula and Medusa Stage
The planula is a larval stage in the life cycle of many bottom-dwelling Cnidarian species. Sperm and egg
cells from medusae form a zygote, which grows into a planula. The planula floats in the currents for a
while, then sinks to the bottom and crawls around looking for a good spot to start a new hydroid colony.
From the hydroid colony, many medusa will develop and be released into the water.
6. Obtain an Aurelia Medusa slide. On the slide, there is usually just one specimen, which has been
stained to show structure. Draw the medusa at a magnification that shows the entire specimen. Label as
many parts as you can.
Q6: What is a planula?
Q7: Draw what a planula looks like.
Use Fig. 13.19 in your book for help.
______X
7. Obtain the preserved Aurelia specimen (A8). On your WS draw and explain its body plan. Include
texture, size, color, etc.
8. Lastly, observe the other cnidarian species: Coral (A9), Sea Anemone (A6)
Draw and explain the body plans of each. Try to label as many parts as you can. (Use Pg. 276-279)
Q8: Explain how a Sea Anemone is different from a Hydra.
Q9: Explain how coral achieves it’s rough skeleton.