Porifera and Cnidaria cont.
Slides and preserved material. Check with your instructor who will use the time remaining as a
guide to see if you have time to work all 5 exercises or she may pick one or two for you to do.
Some pairs should start to tackle sponge cross or longitudinal section slides, others should start
working with cnidarian life cycle slides. This will allow pairs to switch or trade photographs as
we only have three-four slides of each some of the sponge slides.
1. Pairs examining sponge cells: Obtain a slide containing a longitudinal or cross section of Scyha or
Grantia sponge. Try to identify porocytes, pinacocytes, and choanocytes.
Students in the past have found longitudinal sections easier to photograph and label, but slides vary in quality
and again you may have to examine one or two before you decide which to photograph and label.
Reproduced below are the diagrams you used last week to help you identify the different cells.
Another diagram to help you,
2. We should have enough slides of hydra longitudinal sections that all can view these at the same
time but if not, some pairs can start on the exercise .comparing life cycle of different Cnidarians.
Histology: Prepared Slides or whole mount of vitally dyed living specimen. Use the scanning lens of
the light microscope to search the slide and locate the longitudinal section. Once found, use 100X or 200x
to begin your study.
The animal is longitudinally divisible into four regions which should be discernable in your section. At
the oral pole is the hypostome and tentacles.
Below the hypostome is the stomach region. This region accounts for most of the column. Below it is the
shorter, often constricted, stalk. Finally, at the extreme aboral pole, is the pedal disk by which the
animal attaches to the substratum.
Study the histology of the two epithelia in more detail. Cell membranes cannot be seen in these
preparations but nuclei and other organelles may be visible. You may not be able to distinguish among
the cell types present. But there are actually a number of different cell types in each tissue layer.
:
Use the scanning lens of the light microscope to search the slide and locate the longitudinal section. Once
found, use 100X to begin your study.
Allowing for variations due to the location of the plane of section, you should see a cylinder of stained
cells. The cylinder should be closed at both ends. The cylinder is the body column.
The open space in the interior of the column is the coelenteron. It may contain food items and parts of it
may be occluded by the gastrodermis.
The animal is longitudinally divisible into four regions, which should be discernable in your section. At
the oral pole are the tentacles.
Below the tentacles and mouth is the stomach region. This region accounts for most of the
column. Below it is the shorter, often constricted, stalk. Finally, at the extreme aboral pole, is the pedal
disk by which the animal attaches to the substratum.
Use 400X to study the histology of the body wall . This wall consists of two epithelia, epidermis and
gastrodermis, separated by a thin, acellular, connective tissue layer. The connective tissue is the
mesoglea and it should be visible over most of the section as a thin dark line.
Study the histology of the two epithelia in more detail under 200 or 400x. Cell membranes cannot be
seen in these preparations but nuclei and other organelles may be visible. You may not be able to
distinguish among the cell types present. But
there are actually a number of different cell
types in each tissue layer.
:
A longitudinal section through the body
wall of the column of Hydra.
Take a photograph of your stained
preparation of Hydra (400x should prove
best) and try using the descriptions and
diagram, to identify as many different types
of cells as you can.
3. Hydrozoans and Schyphozoans are two groups of Cnidarians that can produce a medusa as part
of the life cycle.
Obtain stained slides of stages of the life cycle of Obelia, a hydrozoan or Aurelia, a sychphozoan. We
also have preserved material. You should at least compare the medusae of Hydrozoans and
Schyphozoans
Work with others in the class to obtain photographs of the different stages in the life cycle.
You may want to set up a central computer that you add photographs to a folder with each pair
contributing a photograph, but everyone using the photographs in their journals to compare the two life
cycles. What are the major differences in the life cycle? Does the Schyphozoan have a colonial polyp
stage? Which medusa appears more complex? Use the following diagrams to aid your survey.
4. Preserved specimens of different “jellyfish may be available. I have ordered a Portuguese man
of war and others. If time permits examine these specimens to see the extreme polymorphism
that can exist in a Hydrozoan colony.
5. We only have a few slides of Metridium, a sea anemone, but if time permits, take a quick look at a
longitudinal or cross section, so you can see how thick in this group the middle layer can get. Note the
more complex mesenteries and extensions of the inner layer.