Phylum Cnidaria Lab
Prepared slides – Hydra whole mount
-hydra with bud
-Obelia medusa
-Obelia hydroids
Demo slide – Metridium sea anemone (c.s)
Demo dissection – Metridium
Demo specimens – Metridium, Aurelia, Gonionemus jellyfish
Preserved samples – Ctenophora (comb jelly)
-jellyfish
-corals
In this portion of the lab, you will exam some of the classes of Cnidarians we studied in lecture
Class Hydrozoa – the hydras & colonial Obelia
Most members of this class exhibit both the hydra and medusoid stages either throughout their life
cycle (Obelia) or only during a part of their life cycle. However, members of the genus Hydra only exhibit
the polyp form.
Exercise 1 – look at the Hydra whole mount under the microscope. Draw the specimen and label the
body stalk, the tentacles and the mouth. Where would the basal holdfast be located? Find the label the
gastrovascular cavity in your diagram. What kind of tissue lines the GV cavity? What kind of tissue covers
the hydra?
Exercise 2 – look at the Hydra with bud slide under the microscope and compare it to what you saw in
exercise 1. Can you find the bud? What kind of reproduction does this bud represent?
Exercise 3 – Look at the Obelia medusoid under the microscope. Draw the specimen in your notebook.
Label the tentacles and the mouth.
Exercise 4 – look at the slide marked Obelia hydroid under the microscope. Locate a polyp and draw
what you see. Label the polyp’s tentacles and mouth.
Demo slide – Obelia colony. Draw what you see under the microscope. Identify the feeding polyps
(hydranths or gastrozoids) and the reproductive medusoids (gonozooids). Identify the stolon of the
colony and the basal holdfast.
Class Scyphozoa – the jellyfishes
The animals in this class are entirely marine. Many of these animals do not exhibit a polyp form and
spend their entire life cycles as the medusoid form
Exercise 1 – Look at the petri dish containing the jellyfishes, Aurelia and Gonionemus. Sketch what you
see and label the tentacles. Can you see the manubrium with the mouth? Can you identify the gonads?
What about the radial and ring canals? Try to identify them and label them on your sketch.
Class Anthozoa – sea anemones and corals
Anthozoan literally means “flowering animals” in reference to the brightly colored forms exhibited by
some members of this class. They exist only as sessile polyp forms. No medusoid form is seen in this
class. The mouth of the Anthozoan known as a sea anemone opens up into a tubular pharynx rather
than a gastrovascular cavity. However, the pharynx is surrounded by and connects to a GV cavity lined
with gastrodermis. Corals secrete a protective calcium based “shell” around them which we see as the
coral bed.
Demo slide – Observe the demo cross section of Metridium and draw what you see. Identify the
epidermis and find the pharynx. Identify the gastrovascular cavity. In cross section, you can see
numerous “walls” called septa that subdivide the gastrovascular cavity. Locate these septa and label
them in your diagram. Many of them contain retractor muscles that can allow the animal to shorten
itself against the substrate if disturbed.
Demo dissection – Observe the dissected Metridium and identify the structures that are labelled on the
diagram next to it.