Porifera and Cnidaria cont.Slides and preserved material. Please do these as time permits. You should have some time the next time we meet to examine these if you run out of time today.

1. Check on your re-associating sponges. Obtain new photographs if there have been changes. Examine the petri dish carefully for attached groups of cells or a layer of cells in some cases. Record your results in your journal.

2. 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,

3. 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. 

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. Tentacles, variously sectioned along transverse, oblique, or longitudinal planes, should also be visible around one end of the column.  The end of the column with the tentacles is the oral pole and an opening, the mouth, may be visible.  In most specimens the mouth is closed and even if open the plane of section probably does not pass through it.

 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 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. 

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.  The mesoglea is little more than the basal laminae of the two epithelia. The two epithelia are monolayered (or pseudostratified) and consists of a layer of cuboidal or columnar cells. 

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.:

* epidermis - single cell layer thick. There are 5 cell types. epitheliomuscular cells - Most common cell type. muscle fibers are longitudinal cnidocytes or cnidoblasts- produce nematocysts interstitial cells - produce new cnidocytes glandular cells - produce mucus nerve cells - most are multipolar * gastrodermis - also single cell layer thick. 5 cell types: nutritive-muscular cells - Most common type. Muscle fibers run circularly. Free ends are flagellated (1 or more flagella) and ameboid. Capable of phagocytosis. nerve cells interstitial cells

glandular cells - secrete proteases and lipases cnidocytes or cnidoblasts (absent from gastrodermis in hydrozoa) * Mesoglea - Stiff jelly-like layer. May be acellular or contain amebocytes. Relatively thin in hydrozoa, but often quite thick in Scyphozoa and Anthozoa.

Figure 2.  A longitudinal section through the body wall of the column of Hydra.

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. Tentacles, variously sectioned along transverse, oblique, or longitudinal planes, should also be visible around one end of the column.  The end of the column with the tentacles is the oral pole and an opening, the mouth, may be visible.  In most specimens the mouth is closed and even if open the plane of section probably does not pass through it.

 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 which you have already examined.

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.  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.:

* epidermis - single cell layer thick. There are 5 cell types. epitheliomuscular cells - Most common cell type. muscle fibers are longitudinal cnidocytes - produce nematocysts interstitial cells - produce new cnidocytes glandular cells - produce mucus nerve cells - most are multipolar * gastro dermis - also single cell layer thick. 5 cell types: nutritive-muscular cells - Most common type. Muscle fibers run circularly. Free ends are flagellated (1 or more flagella) and ameboid. Capable of phagocytosis. nerve cells

interstitial cells glandular cells - secrete proteases and lipases cnidocytes (absent from gastrodermis in hydrozoa) * Mesoglea - Stiff jelly-like layer. May be acellular or contain amebocytes. Relatively thin in hydrozoa, but often quite thick in Scyphozoa and Anthozoa.

Figure 2. 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.

4. 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 medusa of both species. 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.

5. 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. Obtain a photograph of Physalia physalis or Velella and try to identify the different polyps. How does this polymorphism compare to that of Hydractinia?

Velella Note the polymorphism, including the individuals who become the sail in this colony

Physalia physalis is also a colony that has an sail and individuals that are gastrozoids, gonozoids and dactylozoids. The colony moves together, and the perception is that the colony is an individual.

6. 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. Compare the internal structure of a sea anemone with that of hydra.