PHYLUM CNIDARIA – Radially symmetric animals
The phylum Cnidaria is considered. by most zoologists to have diverged from the kingdom Protista independently of the phylum Porifera. Members of this phylum are considered to be more “advanced” than the poriferans for two major reasons. First, they are the first to show tissue level organization, although they have no organs. Second, the adult forms are derived from two distinct embryonic germ layers, the ectoderm and the endoderm (they are diploblastic). Higher phyla are triploblastic (derived from three distinct embryonic germ layers).
The organisms in the phylum Cnidaria are characterized by exhibiting 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 which serves as both mouth and anus, which is often surrounded by tentacles. The body wall has an external cell layer, the epidermis; an internal cell layer, the gastrodermis that lines the gastrovascular cavity; and a layer between the other two, called the mesoglea which may be either cellular, or more often, a cellular. Unique organelles, called nematocysts are found in cells called cnidoblasts. Cnidoblasts are especially abundant on tentacles, but may be generally distributed throughout the epidermis and gastrodermis.
The life cycle of a typical cnidarian alternates between an often sessile polyp stage and a free-swimming medusa stage. The existence of two distinct forms such as this is known as polymorphism. 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. The polyp stage is an asexual stage, while the medusa is a sexual stage. In some cnidarian classes, either the polyp or the medusa stage may be reduced or completely absent.
In this exercise you will examine the three classes of Cnidaria – Hydrozoa, Scyphozoa, and Anthozoa.
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 colony under a dissecting microscope or low power of a compound microscope. Sketch the specimen and label:
a. the hydrocaulus, (main, stalk-like stem) and the perisarc (a transparent, noncellular covering of the stem)
b. a hydranth (feeding polyp) and its tentacles
c. a gonangium (reproductive polyp), and medusa bud
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. Sketch the specimen, labeling:
a. the velum – a circular shelf-like rim attached to the margin of the umbrella and directed inward
b. the manubrium – a dark-colored projection hanging down from the center of the subumbrella cavity. The free end is the mouth.
C. the ring canal, which runs around the circumference of the umbrella
d. the four radial canals, which extend to the margin of the umbrella and connect with the ring canal
e. the tentacles, which arise from the umbrella margin
f. the statocysts (organs of balance), located between the bases of the tentacles
g. the gonads- folded, ribbon-like structures suspended beneath the radial canals
CLASS SCYPHOZOA – Animals in this class are entirely marine. The polyp stage is reduced or absent.
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.
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
c. the pedal disc (basal disc), which is the point of attachment to the substrate