Presentation

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NAME M.UMER ROLL NO bsf2204204SUBJECT
PRINCIPLE OF ANIMAL LIFE TOPIC HYDRA SUBMITTED
TO DR.SANA AZIZ
(Ichthyologist)
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Hydras
Is a genus of small, freshwater organisms of
the phylum Cnidaria and class Hydrozoa. They are
native to the temperate and tropical regions. The
genus was named by Linnaeus in 1758 after
the Hydra, which was the many-headed beast
defeated by Hercules as when the animal had a
part severed, it would regenerate much like the
hydras heads. Biologists are especially
interested in Hydra because of their regenerative
ability they do not appear to die of old age, or
to age at all.
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DIAGRAM
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Scientific classification
Kingdom Animalia
Phylum Cnidaria
Class Hydrozoa
Order Anthoathecata
Family HydridaeDana, 1846
Genus HydraLinnaeus, 17581
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Description
Hydras resemble tiny, delicate, elongated sea
anemones. They have a columnar or trunklike body,
which is structurally a bag (with the mouth at
the top). The mouth is rimmed with several long
tentacles. Hydra bodies are only two cell layers
thick. A sticky secretion at the foot enables
hydras to cling to a surface. Hydras can glide
slowly on their foot, or they can bend over and
turn slow-motion somersaults. Colors vary with
species they can be gray, brown, tan,
cream-colored, green, orange, whitish, pinkish
and clear.
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Morphology

• Hydra has a tubular, radially symmetric body up
  to 10 mm (0.39 in) long when extended, secured by
  a simple adhesive foot known as the basal disc.
  Gland cells in the basal disc secrete a sticky
  fluid that accounts for its adhesive properties.
• At the free end of the body is a mouth opening
  surrounded by one to twelve thin,
  mobile tentacles. Each tentacle, or cnida
  (plural cnidae), is clothed with highly
  specialised stinging cells called cnidocytes.
  Cnidocytes contain specialized structures
  called nematocysts, which look like miniature
  light bulbs with a coiled thread inside. At the
  narrow outer edge of the cnidocyte is a short
  trigger hair called a cnidocil. Upon contact with
  prey, the contents of the nematocyst are
  explosively discharged, firing a dart-like thread
  containing neurotoxins into whatever triggered
  the release. This can paralyze the prey,
  especially if many hundreds of nematocysts are
  fired.
• Hydra has two main body layers, which makes it
  "diploblastic". The layers are separated
  by mesoglea, a gel-like substance. The outer
  layer is the epidermis, and the inner layer is
  called the gastrodermis, because it lines the
  stomach. The cells making up these two body
  layers are relatively simple. Hydramacin4 is
  a bactericide recently discovered in Hydra it
  protects the outer layer against infection. A
  single Hydra is composed of 50,000 to 100,000
  cells which consist of three specific stem
  cell populations that will create many different
  cell types. These stem cells will continually
  renew themselves in the body column.5 Hydras hav
  e two significant structures on their body the
  "head" and the "foot". When a Hydra is cut in
  half, each half will regenerate and form into a
  small Hydra the "head" will regenerate a "foot"
  and the "foot" will regenerate a "head". If
  the Hydra is sliced into many segments then the
  middle slices will form both a "head" and a
  "foot".6
• Respiration and excretion occur
  by diffusion throughout the surface of
  the epidermis, while larger excreta are
  discharged through the mouth.

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Habitat and Conservation
n
Hydras can be found in virtually every kind of
aquatic habitat, though they are most common in
fairly quiet waters such as sunlit pools, where
they attach to submerged vegetation and other
objects, stretch out their tentacles and hunt.
They can occasionally grow to large numbers, and
when this happens in fish hatcheries, they can be
a serious threat to newly hatched fry.
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Food
Hydras wave their tentacles slowly in the water.
When food touches a tentacle, special stinging
cells discharge to help subdue the prey. The
tentacles draw the food into the hydras mouth.
Any tiny animal is fair game, including worms,
small crustaceans, baby fish, young insects and
larval mollusks. One species, the green hydra
(Hydra viridissima), like the saltwater corals
and anemones, holds symbiotic algae in its
tissues and derives nutrients from the algae.
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Life Cycle
Hydras usually reproduce asexually by buddinga
new hydra starts as a bud forming on the side
of a hydras body it grows and eventually breaks
away as a clone of the original. Some species
reproduce sexually, releasing sperm into the
water that can reach eggs on another hydra. The
eggs form a tough coating and can survive drought
and freezing. Sexual reproduction usually only
happens in harsh environmental conditions or
right before winter
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Human Connections
Hydras, though very small, can be kept in
aquariums if you feed them the small worms, water
fleas and miniature, planktonic prey they need.
Unlike saltwater corals and anemones taken from
the wild and sold at pet stores, our freshwater
hydras are numerous and free.
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Ecosystem Connections

• Most of us know that ecosystems are based on the
  tiny plants and animals that form the base of the
  food chain. Hydras form an important link
  between the tiny animals they eat, and their own
  predatorsfish, crayfish and aquatic
  insectswhich are big enough for us to see.

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Nervous system

• The nervous system of Hydra is a nerve net, which
  is structurally simple compared to more
  derived animal nervous systems. Hydra does not
  have a recognizable brain or true muscles. Nerve
  nets connect sensory photoreceptors and
  touch-sensitive nerve cells located in the body
  wall and tentacles.
• The structure of the nerve net has two levels
• level 1 sensory cells or internal cells and,
• level 2 interconnected ganglion cells synapsed
  to epithelial or motor cells.
• Some have only two sheets of neurons.9

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Motion and locomotion

• If Hydra are alarmed or attacked, the tentacles
  can be retracted to small buds, and the body
  column itself can be retracted to a small
  gelatinous sphere. Hydra generally react in the
  same way regardless of the direction of the
  stimulus, and this may be due to the simplicity
  of the nerve nets.
• Hydra are generally sedentary or sessile, but do
  occasionally move quite readily, especially when
  hunting. They have two distinct methods for
  moving 'looping' and 'somersaulting'. They do
  this by bending over and attaching themselves to
  the substrate with the mouth and tentacles and
  then relocate the foot, which provides the usual
  attachment, this process is called looping. In
  somersaulting, the body then bends over and makes
  a new place of attachment with the foot. By this
  process of "looping" or "somersaulting",
  a Hydra can move several inches (c. 100 mm) in a
  day. Hydra may also move by amoeboid motion of
  their bases or by detaching from the substrate
  and floating away in the current

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Reproduction and life cycle

• When food is plentiful, many Hydra reproduce
  asexually by budding. The buds form from the body
  wall, grow into miniature adults and break away
  when mature.
• Hydra budding
• Creating a bud
• Daughter growing out
• Beginning to cleave
• Daughter broken off
• Daughter clone of parent