The phylum Cnidaria includes organisms
with a variety of radially symmetrical forms; all are carnivores. The life cycle of some cnidarians includes both a
medusa (jellyfish) stage and a polyp
(attached) stage. Medusae are
usually free-floating, and often produce gametes.
Polyps are fleshy columns with an attachment at one end and a mouth and
tentacles at the other end. Some
polyps secrete a skeleton made of chitin or calcium carbonate.
Many members of the phylum exist only in one form or the other.
The body wall of cnidarians consists of
two prominent cell layers: an outer
epidermis and an inner gastrodermis.
The mesoglea is found between
the two layers, and may include a thick layer of gelatinous material, which
gives the jellyfish its name. Sme
of the epidermal cells are specialized as protective cnidocytes,
or epitheliomuscular cells which produce movement.
The cells lining the gastrovascular cavity secrete digestive enzymes
which allow digestion of food to be both internal to the organism and
extracellular within the cavity. Although most species of cnidarians are marine, like Gonioniemus
and Metridium that we will examine first, we will also study a common
fresh-water species, the hydra.
a preserved sample of Gonioniemus.
Observe the typical jellyfish body shape, with tentacles ringing the edge
(Perry & Morton, Fig. 88a-b). The
mouth is found in the middle of the ventral surface.
Compare this organism with that that of Metridium,
a marine polyp known as a sea anemone (Perry & Morton, Fig. 90c), because of
its floral-like arrangement. In Metridium,
the tentacles project upwards, the mouth is in the center of the dorsal surface,
and the ventral surface consists of a stalk-like structure that is attached to
rocks or other objects on the seabed, making the adult a sessile
organism. Can you see the similar
arrangement in two organisms which look so different at first?
The hydra is a small cnidarian polyp
that lives in fresh water attached to submerged rocks, leaves and twigs.
Its tentacles stretch out waiting for passing prey.
Obtain a specimen in a depression slide, and examine it under low power,
first allowing it time to relax after being transferred.
Draw a diagram of your specimen, labeling as many structures as possible
(Perry & Morton, Fig. 86). Can
you identify the gastrodermis and epidermis?
Using a needle probe, touch the hydra gently in several places.
Are some areas more sensitive than others? How does the animal respond to this touch?
To observe feeding behavior, transfer
your hydra to a small petri dish, and observe the animal under the dissecting
microscope. You may want to use a
dark background and light the specimen from the side.
A container of small organisms (either the multicellular arthropod Daphnia
or the single-celled protozoan Euglena)
has been provided to feed the hydra. Introduce
a drop containing these organisms to the petri dish with the hydra; and watch
the hydra carefully for the next few minutes.
Record your observations in as much detail as possible.
The capture of prey is normally
followed by a relatively complex series of movements in which the prey is
transported to the widening mouth and then pushed into the gastrovascular
cavity. If your specimen did not respond in this way, watch another
unfed animal capture and ingest prey. These
actions constitute the “feeding
response” and are apparently chemically controlled, although the chemical
message is not completely understood. The
feeding response may also be triggered by a variety of chemical agents, some of
which are present in the bodies of prey animals.
A solution of reduced glutathione is available; it has been identified as
a chemical which is able to trigger this response. Set up a properly controlled experiment to test the hydra’s
reaction to the presence of reduced glutathione. How you use the solution in the experiment is up to you, but
be sure not to add more than a drop or two.
Record your results, including the results of your controls.
Hydra are able to capture prey using a
specialized cell called a cnidocyst,
which contains a harpoon-like nematocyst (Perry
& Morton, Fig. 87a). The
nematocysts may be specialized to penetrate the prey when released, or to
entangle it with sticky threads. Often
a toxin is released along with the nematocyst to assist in subduing the prey.
Attempt to induce a hydra to release its nematocysts by following the
following protocol. Transfer a
hydra to a slide containing a very small amount of water. Add a cover slip and put the slide to the side to dry
somewhat. Observe the hydra at
intervals under the low and high power of the microscope (once discharged, you
will need to use high power to observe them).
Diagram discharged and/or undischarged nematocyts that you see.