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Q: How Do Earthworms
Function?
A:
Most of us are familiar
with earthworms in some
capacity. We might think
of them as fish bait, slimy things
in the ground that come out on the
sidewalk when it rains, slimy things
that are good for the soil, slimy
things that crazy people eat, or the
star of a really bizarre video game
(Earthworm Jim—very strange!).
They’re actually quite interesting,
although you might not realize that
by reading up on earthworm anatomy. Way too many resource articles
focus on naming parts and their
functions, without really explaining what’s going on. Needless to say,
I’m going to do my best to explain
what’s going on instead of boring
you with a bunch of part names, although it’s impossible to stay away
from the names altogether.
Locomotion
You probably think earthworms just
sort of wriggle along, sort of like a
snake. Au contraire. To understand
what they do, imagine you’re an ice
climber with special shoes containing spikes that dig into the ice (these
are sometimes called crampons).
Also imagine you’re not climbing, but rather trying to move your
way across a frozen lake while lying
down. Also imagine you have those
spiked shoes on your hands as well
as your feet. Also imagine you look
pretty silly. Anyway,
here’s one way to get
across the ice. Curl
up in a ball and then
dig the crampons on
your feet into the ice.
Thus anchored, you
uncoil in the direction you want to go.
When you’re completely stretched out,
dig the crampons on
your hands into the
ice. Then release
your feet and curl up
in a ball again. Then
dig the crampons on
your feet into the
ice, release the ones
on your hands, and
start over.
This is almost
what earthworms do
to get through the
soil. They have tiny hairs called setae
on the surface of their bodies. Check
out Figure 1 (p. 70).
They can anchor these hairs into
the soil, just as you can anchor crampons into ice. So, they anchor the
hairs at the rear end of their body in
the soil. Then, instead of uncoiling,
they simply use various muscles to
elongate their bodies. Then they
anchor the setae at the front of their
body in the soil, release the ones at
the back, and shorten their body.
They keep on moving through the
Brian Diskin
By Bill Robertson
soil by elongating and shortening their bodies while anchoring
and un-anchoring the setae. And I
should mention that the elongating
and shortening usually happens a
few segments at a time, rather than
with the entire earthworm body. If
you’ve ever watched earthworms for
any length of time, then you know
that it’s not uncommon for them to
lengthen and shorten periodically.
That would be a nice trick for humans to do. Lengthen for playing
basketball and shorten for flying
coach.
February 2013 69
Figure 1.
clitellum
male pore
setae
Digestive System
The earthworm digestive system
might seem a bit odd because it’s just
one long tube that runs the length of
the body, but it’s really not all that
different from the human digestive
system. If you uncoiled our intestines, our digestive system would
be basically one long tube (and a
long tube it is—about 10 times your
height). Earthworms gather organic
material (decaying leaves or animals
or whatever) in through their mouth
and send it into a storage place
called a crop. From there it goes into
the gizzard, which contains stones
that the worm has swallowed that
grind up the food. This is the same
mechanism birds use. From the gizzard, the material travels on to the
intestine, where enzymes help break
down the food and blood vessels
close to the intestine carry away nutrients. Now, earthworms don’t eat
so they can be beneficial to humans,
70 Science and Children
but that’s the result. Not only do
they aerate the soil with their movements, they bring organic matter
from the surface of the soil down
to lower layers. That makes the soil
better for growing things, and that’s
good for us humans.
Circulatory System
Earthworms are special because
they have five pairs of hearts! Well,
they’re not actually hearts in the human sense but rather aortic arches
that are much simpler in design
than a human heart. They have the
same function, though, which is to
pump blood throughout the worm’s
body. Unlike the human circulatory
system, which has specialized veins
and arteries for going to and from
the heart, earthworms have a closed
system with two main blood vessels through which the aortic arches
pump blood, with many smaller
blood vessels in each segment of
the worm that carry blood back and
forth between the two main blood
vessels. The purpose of a circulatory
system, in all animals, is to get oxygen to different parts of the body.
In humans, the circulatory system
gathers oxygen from the lungs in
exchange for carbon dioxide, but
earthworms don’t have any lungs.
Instead, earthworms absorb oxygen
from the air through their skin and
transfer it to the main blood vessels.
So, their circulatory system is sort
of a long pipe that runs the length
of the body, on bottom and on top,
with a recycled blood supply that
constantly gets replenished with oxygen. Pretty efficient, and they don’t
have to remember to breathe to stay
alive. However, there’s a problem
with too much rain or too little rain.
If the soil is too wet or too dry, earthworms cannot efficiently absorb
oxygen from their surroundings.
That’s why you see earthworms on
sidewalks after rains or in extreme
dry periods. And you thought they
were just coming up to say “hi.”
Reproductive System
Earthworms are pretty quirky when
it comes to reproduction. First of all,
they’re hermaphrodites, meaning
they have the genitalia of both sexes.
So one would think they can simply
reproduce all by themselves, but one
would be wrong. It still takes two
earthworms. The two worms get
side-by-side in opposite directions,
with the appropriate parts lined up.
Each worm deposits sperm in the
other worm, and the sperm and eggs
are held together in what’s called
an oviduct, which is contained in a
structure called the clitellum. After
fertilization, each worm secretes a
ring of mucus around the clitellum,
causing it to fall off. That structure
then is a cocoon for the new baby
worms. It provides enough nutrients
for the baby worms until they break
out of the cocoon and move off on
their own. And unlike my own kids,
the baby worms never come back
home to live for periods of time.
Well, that’s not all there is to
Macro or Micro
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know about earthworms, but it does
give you an idea of just how intricate
these seemingly simple animals are.
Of course, that seems to be what I
discover every time I delve into biology—most organisms are absolutely
amazing in how they function. I also
discover that biologists love vocabulary words, but that won’t surprise
anyone. n
Bill Robertson ([email protected]
com.com) is the author of the
NSTA Press book series, Stop
Faking It! Finally Understanding
Science So You Can Teach It.
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