Members of the phylum Cnidaria are considered to be more "advanced" than poriferans for two major reasons.  First, they show true tissue-level organization of body cells.  Second, the adult forms are derived from two distinct embryonic germ layers, the ectoderm and the endoderm (they are diploblastic).  Most other animal phyla are triploblastic (derived from three distinct embryonic germ layers).


         The organisms in the phylum Cnidaria are characterized by radial symmetry.  Terms for direction, therefore, use the mouth as a point of reference.  The end of the organism which contains the mouth is the oral end; the opposite end of the animal is termed aboral.  Radial symmetry refers to the fact that any plane passing through the oral-aboral axis divides the animal into two equal halves, each a mirror image of the other.


         The basic body plan of the cnidarians is a sac-like structure, with an internal cavity called the gastrovascular cavity.  The gastrovascular cavity has a single opening that serves as both mouth and anus, and it is often surrounded by tentacles.  The body wall has an external cell layer, the epidermis (ectodermal in origin); an internal cell layer lining the gastrovascular cavity known as the gastrodermis (endodermal in origin); and a layer between the other two, called the mesoglea, which may be either cellular, or more often, acellular.


         The life cycle of a typical cnidarian alternates between an often-sessile polyp stage and a free-swimming medusa stage.  Both stages exhibit the body plan described above; however, the polyp stage is cylindrical and attached at the aboral end to a substrate, while the medusa stage is flattened in appearance with the mouth oriented downward.  In some cnidarian classes, either the polyp or the medusa stage may be reduced or completely absent. 



Figure 1.  General body type of cnidarians. From Van De Graaff & Crawley, 1994.



         Unique organelles, called cnidae, are found in cells called cnidoblasts.  Cnidoblasts are especially abundant on tentacles, but may be generally distributed throughout the epidermis and gastrodermis.  One common type of cnida is the nematocyst, a specialized stinging structure used for prey capture and defense.



Figure 2.  Discharged and undischarged nematocysts.  From Margulis & Schwartz, 1998.



         In this exercise you will examine the three classes of Cnidaria: Hydrozoa, Scyphozoa, and Anthozoa. Your instructor will tell you which specimens to observe from each class.





Class Hydrozoa - Members of this class include the genera Hydra, Obelia, and Gonionemus.  Most members of this class exhibit both the polyp and medusa stages; however, Hydra exists only in the polyp form.




1.  Obtain a live specimen of Hydra and observe it under a dissecting microscope.  Draw the specimen, labeling the body stalk and the tentacles.  Place the specimen on a microscope slide with a drop of methylene blue.  Place a cover slip over the specimen and apply GENTLE pressure to the cover slip.  Observe the specimen again, using a compound microscope and looking for discharged nematocysts.  Draw one of these structures. Here are diagrams of an undischarged and discharged nematocyst.





2.  Examine a whole mount of part of a Obelia colony under a dissecting microscope or low power of a compound microscope.  Sketch the specimen and label:


a.  a hydranth (feeding polyp) and its tentacles

b.  a gonangium (reproductive polyp) and the enclosed medusa buds


An Obelia medusa can be viewed here.




3.  Place a Gonionemus medusa in a watch glass and examine its structure under a dissecting microscope.  See if you can tell "which end is up."  That is, locate the upper or convex surface, the exumbrella, and the concave subumbrella.  Note also the velum, a circular shelf-like rim attached to the margin of the umbrella and directed inward; the velum is characteristic of hydrozoan medusa and absent in scyphozoans.  Also note the gonads, opaque ribbon-like structures.  Sketch the specimen, labeling: the velum, tentacles, and gonads.


Class Scyphozoa - Animals in this class are entirely marine.


4.  Obtain and observe a preserved specimen of Aurelia.  Compare Aurelia with Gonionemus.  Note the absence of a velum and that the margin of the bell is scalloped.  You should also see the complex radial canals and the horseshoe-shaped gonads.  Scyphozoan medusae are more complex, with more internal jelly, than hydrozoan medusae.

Figure 3.  Typical scyphozoan, the medusa Aurelia.  From Van de Graaf and Crawley, 1994



Class Anthozoa - the term anthozoa literally means "flowering animals" in reference to the brightly colored forms exhibited by some members of this class.  They exist in the sessile polyp stage only; no medusa stage is present.  The class Anthozoa is a large class whose representatives include the sea anemone and sea coral.  In addition to examining the preserved specimens of sea anemones, you should examine preserved specimens of corals.




5.  Examine a specimen of the sea anemone Metridium.  Draw the specimen and label:


a.  the oral disc and the tentacles attached to it

b.  the mouth (the opening in the oral disc) - the mouth leads into a gullet which is a passageway leading into the gastrovascular cavity.  Look for the siphonoglyph, a vertical groove at the corner of the mouth in anthozoans.  Cilia in the siphonoglyph create a constant flow of water into the pharynx.

c.  the pedal disc (= basal disc), which is the point of attachment to the substrate

Figure 4.  Typical anthozoan, the sea anemone Metridium.  From Van de Graaf and Crawley, 1994.


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