Chapter Five: Phylum Nematoda
The roundworms
Getting to know the roundworms:
If you haven’t guessed by now, this book seems to have a general theme. That theme is discussing
the disgusting ways different animals make their living and this chapter will prove more of the same.
While the vast majority of roundworms are fine, upstanding animal citizens earning their keep as free
living, nonparasitical members of the community that can live in fresh or saltwater or on land, we are not
going to talk about those. Oh no, we will concentrate on the nasty parasites that belong on the fringes of
this phylum including hookworms, ascaris and trichinella.
Roundworm body structure and symmetry:
Now one thing all roundworms have in common despite their lifestyle is the presence of a
pseuodocoelom and a one way digestive tract. A pseudocoelom is a hollow, fluid-filled cavity. It has a
mesoderm lining outside and an endoderm layer inside. You may ask yourself, so what? What does
having a pseudocoelom do for the roundworm. For one, it allows the worm to be round instead of flat
like the acoelomate flatworms. Because these worms have a little more space inside, they have a digestive
tract with two openings. Yes folks, meet the first animal representatives that have an anus. Now you
may not want to contemplate your anus very often but secretly you are probably grateful for it because
now your waste does not need to leave your body through the mouth. But that two opening digestive tract
has more advantages than just being less disgusting. It also allows different parts of the tract to specialize
and does a better job of digesting food. Of course, these critters are also bilaterally symmetrical and
have a definite anterior head and basic cephalization.
On to the less appetizing members of the phylum…….
Ascaris:
These beauties live in the intestines of pigs, horses and humans. It sounds like there are turf wars
going on in our intestines if we aren’t careful. (Remember the other intestinal tenants? The flukes and
tapeworms?) They also feed on your food as it passes by in the intestine. They mate there and produce
up to 200,000 eggs a day that then leave through, you guessed it, your feces! If the eggs fall in a nice
location – dark and somewhat cool – they can live for years waiting. If you eat food or water
contaminated with these eggs, the eggs produce larva that grow up and infest your lungs. You cough
them up and they happily return to your intestines when you swallow them again and grow into contented
adult worms. Did I mention that they can grow up to a foot long?
This mass of ascaris
worms was removed
from a pig intestine.
Yummy.
Hookworms:
I have seen the effects of these lovely specimens first-hand when I was a Peace Corps volunteer in
the Dominican Republic. Before I describe these guys, I have one piece of advice: When your mother
tells you to put shoes on when you leave the house, listen to her. Hookworms have nasty, sharp hooks on
their mouth called cutting plates. They use the hooks to enter in to the body, usually through the soles of
bare feet. They feed on blood and are messy eaters. Those cutting plates aren’t very accurate so they
allow more blood to flow than they eat. How do they get into your body you might ask? Well, since they
enter into your feet from the ground, the eggs have to get into the ground somehow. The hookworms
used that tried and true method of releasing eggs into the feces. If the feces is dropped onto the ground
where it is moist and warm, the eggs produce larva that can then bore into your feet, get into your
bloodstream, head for your lungs where you cough them up and swallow them again so they can develop
into happy little adult hookworms and repeat the process. Whewww. I’m tired. I have to go put on some
shoes now.
Hookworm and hookworm
tracks. Don’t go barefoot!!!!
Trichinella:
The last member of this phylum that we’ll meet is trichinella which kind of sounds pretty but isn’t.
Trichinella is a roundworm that is found in pork and causes the disease trichinosis. They have a similar
life cycle to the other parasitic intestinal worms. They live in the intestine of their host like a pig, they
produce larva that goes to the muscles and goes dormant in a cyst. You eat the muscle tissue with the
embedded cyst, the cyst breaks open and new worms emerge that embed themselves into your intestine
and develop into adults. This can easily be avoided by cooking your meat to a high enough temperature.
The trichinella worm
and its lifecycle.
Cook your meat
thoroughly!
By the time we are done discussing all of these critters, you will a confirmed vegetarian or a huge
supporter of well-done meat!
Photo Credits:
Hookworm: http://www.hsu.com/parasite_large.photo.htm
Hookworm tracks: http://www.scientificpsychic.com/health/hygiene.html
Ascaris worms: http://au.merial.com/producers/pigs/disease/ascaris.html
Trichinella worm: http://www.med.mcgill.ca/tropmed/txt/lecture4.htm
Trichinala life cycle: http://www.mef.hr/Patologija/ch_9c/c9c_trichinella_cycle.html