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Difference Between Hydra and Obelia

Main Difference

The main difference between Hydra and Obelia is that the Hydra consists of polyp body form during its whole life cycle, whereas Obelia consists of both medusae and polyp body form.

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Hydra vs. Obelia

Hydra’s body is flat and tubular with the tentacles around; on the flip side, obelia’s body is upright with branching stems consisting of minute cups. Hydra lives only in the freshwater ecosystem; on the other hand, obelia lives in both the fresh and marine water ecosystems. Only a few hydrae live in the form of colonies; on the flip side, all the obelia live in the colonial form.

Hydra are very small in size of 30 mm long; on the other side of the coin, obelia has a very larger size than hydra consisting of 25cm long. Medusa is absent in the hydra; on the other hand, medusa is present in the obelia. Hydra consists of only sessile or immobile animals, whereas the obelia consists of both sessile or mobile animals.

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Sexual reproduction in hydra usually occurs by the formation of gametes; on the flip side, sexual reproduction in obelia usually occurs in the medusae form only. Hydra does not contain the vast and enormous majority of species in freshwater habitats; on the other side, obelia includes the vast and enormous majority of species in both freshwater and marine water habitats.

Comparison Chart

HydraObelia
Hydra is the coelenterate that has the tubular body with the tentacles all aroundObelia is the coelenterate that has an upright body with branching stems consisting of minute cups
Habitat
Lives only in freshwater habitatLives in both freshwater and marine habitat
Mobility
It remains immobile throughout its lifeIt remains sessile and motile during its life cycle
Sexual Reproduction
It occurs by the formation of gametesIt occurs only in the medusa form
Diversity
It contains only a few species in freshwaterIt contains a vast diversity of species
Body Form
Occur only in polyp formOccur in both polyp and medusa form
Size
It has a tiny size of 30 mm longIts size is longer than hydra that is 25 cm long
Colonial Organization
Only a few occur in colonial formAll occur in colonial form
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What is Hydra?

Hydra considers as the freshwater invertebrate that consists of the stalk-like tubular body with the ring of tentacles all around. The size of the hydra body is very small, which equals 30 mm long, and it is very much smaller. Hydra remains immotile during its whole course of the reproductive life cycle.

It only occurs in the polyp form in the life cycle in which the hydra remains sessile and cannot move in the freshwater habitat. It is the only habitat in which the hydra lives. Hydra lacks highly specialized systems, and the gaseous exchange can occur through the simple diffusion mechanism. Only a few of the hydras live in the colonial organization and all the remaining life as an individual living organism.

Hydra usually reproduces sexually by the formation of gametes and asexually by the formation of buds that grows into the entire new individual by the process of mitosis. A hydra body consists of a mouth as a small hollow opening through which the substances can enter the body of the hydra.

Approximately 6-10 tentacles usually surround the body of the hydra that helps in the capturing of food and insects as prey towards the immotile hydra. Hydra is the diploblastic animal that belongs to the class Hydrozoa having the radial type of symmetry in its body.

What is Obelia?

Obelia considers as the organism of both freshwater and marine water habitat that consist of the upright body posture with the branching stem consisting of minute cups. It is usually sedentary. The size of obelia is very larger than the size of hydra, which equals to the 25 cm long. Obelia does not remain sessile during its whole life cycle, but it also remains motile in the medusa form.

Unlike hydra, it usually occurs in two forms one is a polyp, and the second one is the medusa. It is present in both the freshwater and marine water habitat. It also has a vast and greater majority of species in both the ecosystems. It usually reproduces asexually only in the medusa form by the formation of gametes. It is usually present in the branched tree-like structure, which is known as the sea fur.

It also lacks the highly specialized body mechanisms, and the movement of substances across the body surface only occurs by the simple diffusion mechanism. The obelia is the diploblastic animal having the radial type of symmetry in its body. It usually relates to the class Hydrozoa. It usually made of a hollow structure during its whole extent.

Key Differences

  1. Hydra is the coelenterate that has the tubular body with tentacles all around; on the other hand, obelia is the coelenterate that has an upright body with branching composing minute cups.
  2. Hydra reproduces sexually by the formation of gametes; on the flip side, obelia reproduces sexually only in the medusa form.
  3. Hydra remains sessile in its whole life cycle; on the other side, obelia remains in both sessile and mobile form.
  4. Hydra occurs only in a freshwater ecosystem; on the other side of the coin, obelia occurs in both freshwater and marine water ecosystems.
  5. The size of the hydra is very small; on the flip side, the size of obelia is very larger than the hydra.
  6. Only a few hydras live in the colonial organization; on the other hand, all the obelia occur in colonial form.
  7. There are few species of hydra that are present in the freshwater habitat; on the other hand, a wide variety of species of obelia are present in the freshwater and marine ecosystems.
  8. The size of the hydra is 30 mm; on the other hand, the size of obelia is 25cm.

Conclusion

The above discussion concludes that hydra remains sessile during its whole life, whereas the obelia remains sessile during the first half life cycle and mobile in the next half life cycle.

Janet White

Janet White is a writer and blogger for Difference Wiki since 2015. She has a master's degree in science and medical journalism from Boston University. Apart from work, she enjoys exercising, reading, and spending time with her friends and family. Connect with her on Twitter @Janet__White