hydra vs obelia

Though Hydra and Obelia are both
cnidarians found in the Class
Hydrozoa, there are some
fundamental differences between
Hydra and Obelia. Class Hydrozoa
contains about 3,700 species. All
these species are exclusively aquatic.
However, most of them live in marine
habitats but a few species live in
freshwater habitats
. The common features of Hydra and
Obelia are the presence of tissue level
of organization, radial symmetry ,
mesoglea, tentacles around the
mouth, single opening, which acts as
both mouth and gut, and the absence
of head and segmentation. The most
unique feature of these creatures is
the presence of special stinging cells
called cnidocytes , which contains
nematocytes that are used to capture
prey and for defensive actions. This
article will outline the difference
between Hydra and Obelia.
What is Hydra?
Hydra is a solitary, predatory species
found in freshwater habitats with the
body size of few millimeters long. It
has no medusa stage unlike the other
hydrozoans. It has a cuplike basal
disk that helps to attach to substrates
like rocks, aquatic plants, or detritus.
Both sexual reproduction and asexual
methods can be seen in Hydra. Unlike
the other cnidarians, Hydra has a
remarkable power of regeneration.
Under optimum environmental
conditions, these creatures used to
reproduce asexually by budding . If
the water is immobile, they
differentiate into male and female
and reproduce sexually by producing
gametes .
What is Obelia?
Obelia is a marine cnidarian who
lives on a branched framework as
individual polyps. These creatures are
found in all the marine habitats
except high-arctic and Antarctic
seas. However, these creatures do not
live in deep sea. Obelia has a very
simple body structure with two true
tissue layers; epidermis and
gastrodermis. A jelly like layer called
mesoglea is found in between these
two tissue layers. They have
incomplete digestive system with one
opening where both ingestion of food
and expel of waste is taken place.
Obelia has a simple nerve net with no
brain or ganglia. The life cycle of
Obelia has two stages; motile
medusa and sessile polyp. During
their life cycle, polyps reproduce
asexually by budding and medusa
reproduces sexually by produce
gametes through meiosis. Both polyp
and medusa forms are diploid ,
whereas their gametes are haploid. In
the polyp form, mouth is situated at
the top of the body, surrounded by
tentacles while, in the medusa stage,
the mouth is situated at the distal
end of the body.
Medusa form
What is the difference between Hydra
and Obelia?
• Hydra is a solitary species and lives
attached to substrates, whereas
Obelia is a colonial species and lives
as polyps in an interconnected
branching network.
• Hydra live in freshwater habitats,
whereas Obelia is exclusively marine.
• Hydra does not have medusa form
in their life cycle, whereas Obelia has
both forms; polyp and medusa.
• Unlike Obelia, Hydra has great
regenerative powers.

Author: HandsomebeautyGarden

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