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LESSON 17 E X P L O RI NG THE HYDRA
The
Hydra:
UpOose
and Personal
The Hydra, one of the simplest multicellular
organisms, is a member of the phylum
Cnidaria. The Hydra is a freshwater animal. Its
body is a thin, hollow cylinder with five to
seven tentacles extending from its mouth.
Hydra come in many different colors, including
tan, gray, green, and brown. Adult Hydra are
typically 6 to 13 millimeters long and are capable of ~tretching out or contracting.
•
Adult Hydra
The Hydra's most common method of reproduction is by budding, a form of asexual
reproduction. A small growth, or bud,
forms on the adult Hydra's body
through a series of cell divisions.
The bud soon develops tentacles
and breaks away. The new Hydra
that breaks off can then live
independently.
Hydra also can reproduce by
regeneration, another form of
asexu al reproduction. They can
grow new bodies from small .
pieces that h ave been detached .
When fall arrives and cooler conditions prevail, Hydra usually
develop sex organs and reproduce
sexually. This is because th e eggs that
Hydra budding
(continued)
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LESSON 17 EXPLORI N G THE HYDRA
are produced are able to survive the colder
conditions, while the parents cannot.
The Hydra has no circulatory system with a
heart and blood vessels. B.ecause its body is
only two cell-layers thick, the Hydra's cells
can get the nutrients it needs through a
process called "diffusion." This occurs as the
Hydra moves through the water and the cells
interact with the environment, exchanging
oxygen and food for carbon dioxide and other
Epidermal cells
wastes through the cell membranes.
The Hydra's outer layer of cells protects the
organism. Its inner layer of cells secretes
enzymes that digest the Hydra's food.
The Hydra does not have a central nervous
system. Instead, it has a "nerve net" through
which impulses that control the Hydra's muscle
contractions are carried. These contractions
allow the Hydra to expand, contract, and move
about.
Sting cell
Nematocyst
I,
Sting cell with
t hread of
nematocyst
discharged
I
.......
. ....)
Notice the coiled thread inside the sting cell on the left. It has been discharged from the sting cell on the right.
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LESSON 17 EXPLORI NG T H E HYDRA
Paralyzing Poisons
and Tight Embraces
le
e
lith
The Hydra feeds on
small organisms,
such as Daphnia.
Specialized cells that
make up part of the
outer layer of a
Hydra contain stinging structures called
"nematocysts."
When an organism
such as Daphnia
brushes against the
Hydra's tentacles,
This Hydra has captured and paralyzed a minnow. Is it biting off more than it can chew?
the nematocysts
shoot out threads.
These threads pierce
the prey and release a paralyzing poison.
digestive cavity, wave about to stir the digestive
Other nematocysts release threads that surfluid. Undigested pieces are released through
round the prey and hold it tight. The tentacles
the Hydra's mouth. Digested nutrients move
then,_ move the prey toward the Hydra's mouth
into the cells of the Hy dra by diffusion.
and force it into the its digestive cavity.
The Hydra spends much <?f its time attached
The Hydra secretes digestive enzym es into
to a surface by its base. However, when it's time
to move , the Hydra moves in style . It simply
this cavity from cells in the inner layer. These
floats or travels from one location to another by
enzymes break down the prey into usable
doing somersaults! D
nutrients. Whip-like flagella, which line the
The Hydra is somewhat of a gymnast, often performing somersaults to move from one place to another.
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