Obelia

            Obelia is a colonial marine organism that is found attached to seaweeds, rocks, shells or pilings in the shallow waters off seacoasts.  This animal alternates between a polyp stage (which is the asexual generation) and a medusa stage (which is the sexual generation). 

Examine Figure 4 that illustrates the life cycle of Obelia.  The plantlike colony consists of numerous branches, which terminate in two kinds of polyps.  The feeding, or nutritive polyps possess tentacles and resemble Hydra.  The reproductive polyps are club-shaped and lack tentacles.  The branches of the colony are covered by a transparent sheath - the perisarc, which extends around the nutritive and reproductive polyps as the hydrothecae and the gonothecae, respectively.

Examine slide #63 of a hydroid colony of Obelia and slide #62 of a medusa of Obelia sp.   

                                                Figure 4: Life cycle of Obelia sp.

 

       

 Figure 5:  Different microscopic views of Obelia sp.

Obelia sp. colonies.

A - new polyp forming, B - reproductive polyp, C- feeding polyp.

Obelia sp. medusa.

Examine the medusea on demonstration of the common jellyfish, Gonionemus (Figure 6).  The medusa is umbrella-shaped and swims with its convex surface upward so that the tentacles are facing downward into the water.  The tentacles have numerous nematocysts to assist in capturing prey.  Extending into the cavity of the medusa is the manubrium, which contains a mouth at its tip surrounded by four oral lobes.  Food organisms enter the mouth and into a gastrovascular cavity. The gonads (testes or ovaries) can also be seen on the undersurface of the medusa.  Gonionemus sp is dioecious; that is it has the male and female organs in separate organisms.   The eggs and sperm are released into the sea where fertilization takes place.  The zygote develops into a planula larva, which swims about, then settles, and is transformed into a polyp.

Figure 6: Medusa of Gonionemus sp. (a) and diagram showing the structures of medusa (b).