Activity 36
Name
Period
Lab Partner
EXAMINING THE ASCARIS
PURPOSE:
To understand degeneration as an adaptation to a particular environment.
To understand parasitism.
To define "host" as it relates to parasitism.
To understand that many parasites have complex life cycles.
To understand why many parasites have evolved complex and efficient
reproductive systems over time.
MATERIALS PER LAB STATION:
one preserved female Ascaris one dissection tray
two dissecting probes
one razor blade
several dissecting pins
one pair forceps (lab "tweezers")
latex gloves (if the student chooses to bring them)
Introduction:
In general, there are three major types of worms. These include the flatworms (Phylum
Platyhelminthes), the round worms (Phylum Nematoda), and the segmented worms (Phylum Annelida),
of which the earthworm is a common example. In this dissection, you will be examining a nematode
worm of the genus Ascaris.
Nematode worms are a diverse and abundant group. A shovelful of garden soil would contain millions
of them. Over 50 different species have been found in humans. Some roundworms occupy very specific
habitats. One species occurs only in the human appendix.
Many nematode worms are free-living while many others are parasitic. A parasite is an organism that
lives in close association with another organism (its host), inflicting harm on the organism with which it
is associated. Some parasites are internal, like the Ascaris, living within its host's intestines. Some
parasites, like fleas and ticks, live on the exterior of their hosts. While a parasite, by definition, is
harmful to its host it is important to understand that selection would favor those parasites that don't kill
their hosts. Think why this would be true.
Nematode worms do not creep along a surface to move. Because they only have muscle fibers that
extend the length of their body (longitudinal muscles) and have no circular muscles, they have a
thrashing pattern of movement. In water this thrashing about does not really allow them to move
forward. Inside the soil or intestines, however, there is enough friction provided by particles to allow
them to move forward.
Nematode worms have cylindrical bodies that are pointed at both ends. They have no cilia inside or out.
Their bodies are covered with a thick tough cuticle secreted by their outermost layer of cells.
The mouth of the worm is at the anterior end of the worm, and allows food to enter a muscular
pharynx. It is the pharynx that actually provides the force to suck in food. The pharynx then leads to a
long intestine that extends the whole length of the worm. The intestine is only one cell layer thick. The
intestine will appear quite flimsy and collapsed in your specimen. The wastes exit the intestine through
the anus at the posterior end of the worm. In nematode worms that are not parasitic, there are gland
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cells in the intestine that secrete digestive enzymes to break down the food. In parasitic species that live
in the intestines of animals, such as the one you will be dissecting, there are no gland cells in the
intestine. Can you figure out why?
In nematode worms, the sexes are separate, with the males being smaller than the females. You will be
dissecting a female worm. The reproductive system consists of 2 long tubes that eventually join to form
a Y-shaped structure (see the accompanying figure). Each tube coils within the body cavity. At the end,
each tube is very thin and highly coiled. These are the ovaries. The ovaries produce the eggs. The tubes
widen as they approach the place where they join together. The thicker area of the tubes are the
uteruses, where the eggs accumulate. The two uteruses join to form a Y. The single tube after the two
uteruses join is the vagina, which leads to the genital pore. The eggs pass from the uteruses into the
vagina and out the genital pore, where they will then continue the life cycle of the worm.
Ascaris, like many parasitic worms, shows a rather complex life cycle. Nearly 200,000 eggs are laid by
the female worm each day while she resides in the intestine of the host animal. These eggs travel out of
the host in its feces. Inside the eggs, little worms start developing. Even with this large number of eggs,
many of the little worms will not survive to reach the next stage of the life cycle. Many of the
developing worms die if the eggs are subjected to drying or if the temperature falls below 60 degrees F
or rises above human body temperature (about 98-99 degrees F). If a human happens to eat food or
drink water that contains a viable egg, the egg will pass through the harsh stomach unharmed and will
hatch inside the intestine. Rather than staying in the intestine, and developing there into the adult, the
little worms travel throughout the body of the host. It is this activity that is most harmful to the host.
The small nematodes first bore through the wall of the intestine into blood vessels, where they get
carried to various organs. The worms only remain in the lungs, however, where they bore through the
lungs into the bronchial tubes. From there, they crawl up the windpipe (trachea) eventually into the
pharynx, where the host swallows them. They pass through the stomach into the intestine, where they
develop into adult worms, where they can mate and lay more eggs, continuing the cycle. While the
worms are living they secrete substances which prevent the host's digestive enzymes from killing them.
Should the worms die, however, the host digests them.
Adult Ascaris worms in the intestines of humans is relatively harmless unless they occur in large
numbers. In large numbers they may cause a blockage of the intestines, killing the host. As many as
5000 adult worms have been found in a host at one time.
When examining your specimen during this dissection, it is important to note the LACK of many organs
inside the worm. Most of the organs you will see are part of the reproductive system. At first, this
apparent simplicity may diminish our "impression" of this organism. Do not, however, assume that a
lack of complexity means being "less advanced." In fact, the Ascaris is showing an adaptation called
degeneration. Organ systems become degenerate if they become simpler or more reduced over time
(instead of more elaborate or complex). Some organs or organ systems may eventually be lost
altogether. This is the case with nematode worms. Parasitic worm ancestors were free-living, and
therefore needed more complex organ systems in order to live. The first parasitic worms were
preadapted to survive in the intestines of a host, and over time evolved to better survive in the new
environment. The worms started to lose organs or systems that were of no use in the new environment.
Given this, we should see that degeneration is one way in which certain organisms adapt to a new
environmental situation. It does not only occur in parasites. The following example shows how
degeneration could occur in a free-living group of organisms. If a group of organisms became
established in a cave that had no light, it would be an adaptation to lose their eyes. After all, why would
it be beneficial to have them when they require energy to maintain, and are prime places for infection. It
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would be wrong for us to look at this new population and to assume they were less advanced than
species with eyes.
A major advantage of degeneration is that it allows the organism to spend more of its energy in other
areas. Also, if now-useless organs are lost, it allows more space for other organs to expand or develop.
In Ascaris, the reproductive organs have become more prominent, allowing the female worm to lay over
200,000 eggs daily.
Figure 1. Ascaris anatomy.
PROCEDURE:
1. Pin your Ascaris to the dissecting pan at its anterior and posterior ends.
2. Using your dissecting probe, slit the organism along its midline from anterior to posterior.
3. Place pins to hold the sides of the worm open to view.
4. Using the Figure on the previous page and the description in the introduction, locate the intestine,
ovaries, uteruses, and vagina.
5. Insert a pin under the muscle fibers that entend down the body wall. Pull the pin upward and note
that
the muscles run in only one direction (longitudinally, not circularly). This limits the movementof
all roundworms (nematodes).
6. Clean up your lab station according to the instructions of your teacher.
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Lab: Examining Ascaris
Name ________________________________
PRELAB QUESTIONS: In order to answer these questions you will need to read through the entire lab:
introduction, procedure, and materials list. After you have carefully read all of the lab, answer these
questions.
1. What is a parasite?
2. Where are the eggs produced in a female Ascaris?
3. What is degeneration?
4. Will you be dissecting a male or female worm?
5. Which piece of equipment are you going to use to open your worm?
6. Using a dictionary, if needed, differentiate between "anterior" and "posterior."
ANALYSIS QUESTIONS:
Answer the following questions in complete sentences.
1. Why is the Ascaris reproductive system so highly developed?
2. One benefit of degeneration in the Ascaris is that it can remain small. Why would this be so
beneficial for this organism?
3. The eggs of Ascaris has shell that is so resistant that the embryo can continue to develop for a time
even when they are placed in concentrated solutions of dangerous chemicals. Explain how this
resistant shell is an adaptation to theAscaris's parasitic lifestyle.
4. The intestine of free-living namatodes has gland cells which secrete digestive enzymes to break
down food. The Ascaris's intestine has lost these cells. Explain why this may have happened.
5. Explain how degeneration is an adaptation.
6. Why would it be detrimental for parasites to kill their hosts?
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