BIOS 173 
Lab 2 
Phylum Cnidaria


 Cnidarian Taxonomy

Within the animal kingdom the jellyfish, sea anemones and corals make up the phylum Cnidaria. Distinguishing characteristics of this group are:

  1. radial symmetry in both the polyp and medusa body types;
  2. most species exhibit polymorphism and their life cycles involve an alternation between the polyp and medusa forms;
  3. the presence of cnidocytes containing stinging threads called nematocysts (or cnida) from which the phylum gets its name;
  4. tentacles around the mouth;
  5. a nerve net providing a modest degree of coordination;
  6. a tissue level of organization characterized by an outer ectoderm, an inner endoderm and a layer of jelly-like mesoglea sandwiched between;
  7. a relatively large gastrovascular cavity.
If you have ever snorkeled or dived on a living coral reef, you have seen a virtual jungle of cnidarians. Most of the species are marine, but, there are a few freshwater species belonging to Class Hydrozoa, such as Hydra, which are present at Stroud's Run and in local ponds. The freshwater jellyfish pictured on Buchsbaum p. 119 has also been found in ponds and small lakes in Ohio (see article from ODNR newsletter in lab).

Three classes of cnidarians are described in Buchsbaum pp. 111-147. Class Hydrozoa is characterized by animals that usually have both polyp and medusa forms in their life cycle (Hydra an exception). All the medusa forms have a velum, a thin, muscular ring of tissue along the inner margin of the bell that aids in swimming. Representative genera you are responsible for are Obelia, Gonionemus, Hydra, and Physalia.

Members of Class Scyphozoa include the larger marine medusae forms. The velum is lacking in this group. The polyp stage is typically very small or absent from the life cycle. Be able to recognize and classify the genus Aurelia.

The sea anemones, all the corals, sea fans, plumes and whips belong to Class Anthozoa. These are solitary or colonial animals with a polyp stage only, there is no medusa form. The polyp is different from those in other classes because the gastrovascular cavity is partitioned. The only genus you need remember is our representative sea anemone, Metridium. The corals and sea fans and sea whips on display need only be categorized to class.
 

Hydra Appearance and Behavior (see Buchsbaum pp. 88-93)

After viewing the film loop demonstrating Hydra behavior, examine the living specimen in the dish at your table. Place the dish on the stage of a stereomicroscope and locate the following structures on your specimen:

  1. basal disc at the lower end which serves for attachment to the substrate;
  2. the cylindrical body;
  3. count the tentacles encircling the free end;
  4. the mouth.
Check also for the presence of any buds, or developing egg cells or testes on this individual.

Check the table in front of the room for a container of Hydra that have been fed water fleas or brine shrimp. Take notes as you observe prey capture and feeding.
 

KINGDOM ANIMALIA: Phylum Cnidaria

Class Hydrozoa (marine and freshwater)

Class Scyphozoa (marine only) Class Anthozoa (marine only)

For More Information on Cladistics

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