What are the Structural Differences Between Obelia and Jelly Fish

The dominant phase of the life cycle of Obelia is a polyp whereas the dominant phase of the life cycle of jellyfish is a medusa. Furthermore, the dominant phase of the life of Obelia is very small while the dominant phase of the life of jellyfish is large. These are the main structural difference between Obelia and jellyfish.

Obelia and jellyfish are two classes of the phylum Cnidaria. Generally, cnidarians are relatively simple invertebrates that are exclusively

Key Areas Covered

1. Obelia
     – Definition, Structure, Life Cycle
2. Jellyfish
     – Definition, Structure, Life Cycle
3. What are the Structural Similarities Between Obelia and Jellyfish
     – Outline of Common Features
4. What is the Structural Difference Between Obelia and Jellyfish
     – Comparison of Key Differences

Key Terms

Cnidaria, Jellyfish, Medusa, Obelia, Polyp

Obelia – Definition, Structure, Life Cycle

Obelia is a genus that belongs to the class Hydrozoa classified under the phylum Cnidaria. It contains relatively simple forms of animals that are also known as sea fur. Most Obelia are marine while a few of them live in freshwater. They live no deeper than 200 meters (660 ft) from the water’s surface.

Figure 1: Obelia

Significantly, Obelia undergoes alterations of life cycle between polyp and medusa stages. However, the dominant phase of the life cycle is the polyp phase, which is asexual. The polyp stage is sessile and colonial. Individual hydranthan in the mature colony is known as a gastrozooid, which has a medusa bud. After that, the medusa stage sexually reproduces through the production of sperms and eggs. Ultimately, the fertilized egg gives rise to a zygote, a blastula, and then to a ciliated, swimming larva known as a planula.

Jellyfish – Definition, Structure, Life Cycle

Jellyfish is a group of animals that belong to three classes including Cubozoa (box jellyfish), Scyphozoa (true jellyfish), and Staurozoa (stalked jellyfish). Additionally, they collectively belong to the phylum Cnidaria. Although jellyfish undergo an alteration of life phases between a polyp and medusa, their dominant phase of the life cycle is a medusa. Furthermore, this medusa form is a large, free-swimming, marine animals with umbrella-shaped bells with tentacles.

Figure 2: Jellyfish

On the contrary, jellyfish can live from the surface of the sea to the deep sea. Also, true jellyfish is exclusively marine. Large and colorful jellyfish can be found in the coastal regions. Their sessile polyp phase follows the planula larva developed through the sexual reproduction of the medusa phase.

Structural Similarities Between Obelia and Jellyfish

• Obelia and jellyfish are two classes of the phylum Cnidaria.
• Their life cycle alters between polyp and medusa stages.
• Here, their medusa phase is free-swimming while the polyp phase is sessile.
• Furthermore, the medusa phase is the sexual phase while the polyp phase is the asexual phase.
• Also, both life stages have radial symmetry.
• Besides, their mouth is surrounded by tentacles, which bear cnidocytes.
• Moreover, they have a single body cavity used for digestion and respiration.
• They are diploblastic animals with mesoglea sandwiched between two layers.
• Additionally, their nervous system is composed of a nerve net and simple receptors.

Structural Differences Between Obelia and Jellyfish

Definition

Obelia refers to a genus of sedentary colonial coelenterates with upright branching stems bearing minute cups in which the polyps sit while jellyfish refers to a free-swimming marine coelenterate with a gelatinous bell- or saucer-shaped body that is typically transparent and has stinging tentacles around the edge.

Dominant Phase of the Life Cycle

The dominant phase of the life cycle in Obelia is the polyp while the dominant phase of the life cycle in jellyfish is the medusa.

Structure of the Dominant Phase of the Life Cycle

Moreover, the polyp phase of Obelia has a vase-shaped body while the dominant phase of jellyfish has a bell-shaped body. Thus, this is one of the major structural differences between Obelia and jellyfish.

Size of the Dominant Phase of the Life Cycle

Also, another of the structural differences between Obelia and jellyfish is the size of the dominant phase of the life cycle. That is; the dominant phase of the life cycle of Obelia is very small,  the dominant phase of the life cycle of jellyfish is large.

The sexuality of the Dominant Phase of Life Cycle

Furthermore, the dominant phase of the life cycle of Obelia is asexual while the dominant phase of the life cycle of jellyfish is sexual.

Polyp Phase

Polyp phase is also one of the structural differences between Obelia and jellyfish. The polyp phase of Obelia produces branched stalks while the medusa phase of jellyfish produces unbranched stalks.

Conclusion

Obelia is a form of hydrozoans that have branched stalks. Also, the dominant phase of the life cycle of Obelia is the polyp. However, the polyp form of Obelia is very small. On the other hand, a jellyfish is a form of cnidarians whose dominant phase of the life cycle is a medusa, which is comparatively large. Furthermore, Obelia and jellyfish belong to separate classes under the phylum Cnidaria. These are the main structural differences between Obelia and jellyfish.

References:

1. “28.2: Phylum Cnidaria.” Biology LibreTexts, Libretexts, 8 June 2018, Available Here.

Image Courtesy:

1. “Obelia geniculata” By Haeckel_Campanariae.jpg:Ernst Haeckel(1834–1919) (Public Domain) via Commons Wikimedia   
2. “Cross section jellyfish en” By Mariana Ruiz Villarreal LadyofHats – Made myself using these sources:[1],[2],[3], [4], Image:Hydromedusa.svg, [5], Olindias formosa. (Public Domain) via Commons Wikimedia