Question

What are polyp and medusa? What is their use? Explain their structures in Obelia.

Answer 1

polyp and medusa, names for the two body forms, one nonmotile and one typically free swimming, found in the aquatic invertebrate phylum Cnidaria (the coelenterates). Some animals of this group are always polyps, some are always medusae, and some exhibit both a polyp and a medusastage in their life cycle.

The polyp is a sessile, or nonmotile, organism; well-known solitary polyps are the sea anemone and the freshwater hydra. The medusa, when free swimming, is popularly known as a jellyfish.

Anatomy

The two forms are similar in construction; both consist of a cylindrical body surrounding a digestive cavity, with a single opening, the mouth, at one end.

The mouth is surrounded by tentacles, which are used to capture food and convey it to the mouth; these tentacles are armed with stinging cells which paralyze the prey. The body wall is composed of three layers of tissue. Thin layers called endoderm and ectoderm line the outside and inside, respectively; between these is a layer of jellylike material, called mesoglea, of varying thickness.

The polyp, also called the hydroid, tends to be elongated, with a thin body wall; it is attached to the ocean bottom or other surface by the end opposite the mouth, its tentacles pointing upward. The medusa tends to be rounded, with a thick body wall containing much mesoglea; it swims or is carried in the current with the mouth side down and the tentacles dangling. 

Reproduction

In organisms that exhibit both forms, such as members of the cosmopolitan genus Obelia, the polyp is the asexual stage and the medusa the sexual stage.

Polyp versus Medusa :

1) Polyp is a sessile life cycle stage of the Cnidaria phylum.Medusa is a mobile life cycle stage of the Cnidaria phylum, contracting with it muscular bell.

2) Polyp have a tubular shape and are fixed at their base, with the mouth present at the other end of the tube facing the water.Medusa have a bell shape, with tentacles hanging down.

3) Polyp do not have a manubrium.Medusa of the Hydrozoa class present a tube hanging down from the bell known as manubrium.

4) Polyp do not possess any sense organs.Medusa possess photoreceptors and gravity-sensing statocytes surrounding the bell.

5) Polyp can reproduce asexually by budding, or sexually by spawning following the release of pheromones.Medusa reproduce exclusively sexually.

6) Polyp produce either polyp or medusa by budding.Medusa can produce only medusa.

7) Polyp are primitive, being sessile, lacking sense organs, and reproducing mostly asexually.Medusa are more evolved, being mobile, presenting sense organs such as the photoreceptors and the statocytes, and reproducing sexually which promotes genetic diversity.