Download 07 Lab - Worm Disection

Earthworm Dissection Lab
Background:
Among the most familiar invertebrate animals are the earthworms, members of the phylum
Annelida. The word annelida means "ringed" and refers to a series of rings or segments that
make up the bodies of the members of this phylum. Internally, septa, or dividing walls, are
located between the segments. External segments are called metameres. There may be
more than 100 segments in an adult worm. The clitellum is a swelling of the body found in
sexually mature worms and is active in the formation of an egg capsule, or cocoon. Eggs
are produced in the ovaries and pass out of the body through female genital pores. Sperm
are produced in the testes and pass out through tiny male genital pores. During mating,
sperm from one worm travel along the sperm grooves to the seminal receptacles of another
worm. Fertilization of the eggs takes place outside the body as the cocoon moves forward
over the body, picking up the eggs of one worm and the sperm of its mate. The pumping
organs of the circulatory system are five aortic arches. Circulatory fluids travel from the
arches through the ventral blood vessel to capillary beds in the body. The fluids then collect
in the dorsal blood vessel and reenter the aortic arches. The earthworm takes in a mixture
of soil and organic matter through its mouth, which is the beginning of the digestive tract.
The mixture enters the pharynx, which is located in segments 1–6. The esophagus, in
segments 6–13, acts as a passageway between the pharynx and the crop. The crop stores
food temporarily. The mixture that the earthworm ingests is ground up in the gizzard. In the
intestine, which extends over two-thirds of the body length, digestion and absorption take
place. Soil particles and undigested organic matter pass out of the worm through the rectum
and anus. The nervous system consists of the ventral nerve cord, which travels the length of
the worm on the ventral side, and a series of ganglia, which are masses of tissue containing
many nerve cells. The nerve collar surrounds the pharynx and consists of ganglia above
and below the pharynx. Nervous impulses are responsible for movement and responses to
stimuli. Each segment contains an enlargement, or ganglion, along the ventral nerve cord.
Excretory functions are carried on by nephridia, which are found in pairs in each body
segment. They appear as tiny white fibers on the dorsal body wall. The earthworm has no
gills or lungs. Gases are exchanged between the circulatory system and the environment
through the moist skin.
Earthworm Dissection
Earthworms are important helpers in the garden or field! Their
tunneling mixes up the soil and brings rich soil to the surface. You can
observe the organs of these tiny creatures by dissecting a preserved
earthworm.
Observation: External Anatomy
1. Find the anterior (front) end of the earthworm by locating the fleshy
bump over its mouth, called the prostomium. The posterior (back)
end has a small hole where solid waste is expelled, called the anus.
The length of the worm is made up of many tiny segments, each
separated by a thin wall called a septum.
2. About one-third of the way back from the mouth you should see a
thicker and smoother section of the worm. This is called the clitellum,
and it is involved in reproduction.
3. Notice that the earthworm has a rounded dorsal (back) surface and a flatter ventral
(belly) surface. Usually the dorsal surface is darker than the ventral surface (though
sometimes this is obscured in the preservation process). Lightly rub your finger along the
ventral side toward the posterior end of the worm. You should feel a roughness caused by
tiny bristles called setae. Using a magnifying glass, try to see the setae.
4. With your magnifying glass look for tiny pores on each segment. Liquid wastes are
expelled through these pores. Near the front end of the worm you should see some larger
pores that can be easily seen without magnification. These are genital pores and are
important in reproduction.
Dissection: Internal Anatomy
1. Lay the worm on your dissecting tray with its dorsal side facing up. Use dissection pins to
secure each end on the tray. Start your dissection about an inch posterior to the clitellum.
Lift up the skin with a pair of forceps and snip an opening with a pair of dissecting scissors.
Insert the scissors into the opening and cut in a straight line all the way up through the
mouth. Go slowly and be sure to cut just the skin--if you go too deep you may damage the
internal organs.
2. Using the forceps and dissection pins, carefully pull apart the two flaps of skin and pin
them flat on the tray. (You may need to drag a pin along the inside of the skin to sever the
septum walls to make it easier to spread the skin.)
3. Look at the labeled picture to help you find the following
features:
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Pharynx: This is the light-colored organ just inside
the mouth. Its muscular contractions pass food on
down to the esophagus.
Hearts (or "aortic arches"): Behind the pharynx are
five dark loops wrapped around the esophagus.
Click for full-size pdf
These are the blood vessels that serve as the hearts
of the worm.
Dorsal blood vessel: This is a dark line extending from the hearts over the top of
the crop.
Crop: Food from the esophagus is temporarily stored in the crop.
Gizzard: Food comes from the crop into the gizzard, where it is ground up.
Intestine: The intestine is the long tube extending from the gizzard all the way to the
anus. Food is digested and absorbed here.
Reproductive organs: The light colored tissue above and around the hearts are
seminal vesicles. Other reproductive parts appear as small white organs on the
ventral side of the hearts.
Ventral Nerve Cord: With your forceps, gently push aside the intestine to view the
long white nerve cord running along the length of the worm beneath it.
4. Optional: Finish cutting the rest of the worm open from the first incision through to the
anus. Observe how the intestine and ventral nerve cord both continue through the entire
length of the worm.
Name(s)_______________________________ Date__________ Period_______
Earthworm Lab sheet
1. Which part of the worm uses muscular contractions to pass food from the mouth
to the esophagus?
2. How many hearts are there in a worm?
3. Where is food stored temporarily?
4. What happens in the gizzard?
5. Where does the intestine start and end?
6. Where does digestion and absorption take place?
7. What happens to the undigested soil particles?
8. Your dissection of the earthworm did not go beyond segment 32.What will you
observe if you dissect the remainder of the worm to its posterior end?
9. Label the parts of the earthworm you observed.
10.
What are the similarities and differences between the human digestive
system and the worm’s digestive system. (Answer should be a paragraph of at
least 5-6 sentences or a chart)
Rubric for Worm Dissection
Categories
Knowledge /
Understanding
-use appropriate
terminology related to
digestion
Thinking and
Investigation
-conduct inquiries
involving locating and
describing the
interrelationship
between digestive
organs in a worm
Communication
-expression and
organization of ideas
and information (e.g.,
clear expression, logical
organization)
Level 1
(50 - 59%)
Level 2
(60 - 69%)
Level 3
(70 - 79%)
Level 4
(80 - 100%)
-demonstrates
limited
understanding
of terminology
-demonstrates
some
understanding of
terminology
-demonstrates
considerable
understanding of
terminology
-demonstrates
thorough
understanding of
terminology
-conducts
inquiries with
limited
effectiveness
-conducts inquiries
with some
effectiveness
-conducts inquiries
with considerable
effectiveness
-conducts inquiries
with a high degree
of effectiveness
-expresses and
organizes
ideas and
information
with limited
effectiveness
-expresses and
organizes ideas
and information
with some
effectiveness
-expresses and
organizes ideas
and information
with considerable
effectiveness
-expresses and
organizes ideas
and information
with a high degree
of effectiveness
Demonstrates
poor safety
management
skills (i.e. rarely
wear s safety
goggles, does
not keep work
area organized
...)
Demonstrates fair
safety
management skills
(i.e. occasionally
wears safety
goggles, does not
keep work area
organized . . . )
Demonstrates good
safety
management skills
(i.e. always wears
safety goggles,
work area is
usually neat and
organized . . . )
Demonstrates
excellent safety
management skills
(i.e. always wears
safety goggles,
work area is
always neat and
organized . . . )
Demonstrates
poor laboratory
skills
Demonstrates fair
laboratory skills
Demonstrates good
laboratory skills
Demonstrates
excellent laboratory
skills
Application
Safety
Laboratory Skills