Earthworms are ideal specimens to use for teaching basic anatomy and
investigating simple organ systems. They also provide an economical and
convenient opportunity for students to practice their dissection techniques
before progressing to more complex specimens such as frogs or pigs. Use the
following procedure and your students will be “digging into earthworms” in no
time.
Earthworm dissection procedure
Observe the external anatomy of the earthworm. Earthworms are
annelids, or segmented worms that have bodies made up of a series of
ringlike segments. The earthworm has no head and no appendages, but
does possess external characteristics to study. First, identify the
anterior and posterior ends. The anterior end is somewhat larger
than the posterior. Pin the earthworm to the dissecting pan and
count and record the number of segments. There may be as many as
100.
Observe each segment closely. Note the tiny bristles on the
ventral surface. These setae help the worm move through soil.
Each segment also contains a pair of small excretory pores
called nephridiopores. You may need to use a hand lens or
stereomicroscope to see these openings.
Openings to the oviducts are found at segment 14, counting from
the anterior end. These female pores release the eggs. Sperm ducts
can be found on segment 15. The clitellum is the enlarged structure
that begins at segment 31. The clitellum secretes mucus that holds 2
earthworms together during mating. It also produces a cocoon in
which eggs and sperm are deposited.
Place the earthworm on its ventral side. (The ventral side is
more flattened than the dorsal side.) Using a scalpel, make a
shallow incision anterior to the clitellum and continue the incision
toward the mouth. Be careful not to cut too deep or you will slice
into the digestive system. Using forceps, spread the incision open
and pin the body wall to the dissection pan as shown in the
illustration. You may choose to add some water to the specimen to
prevent it from drying.
Identify the thin walls between each segment. These are called
septa.
Identify the organs of the digestive system. Beginning at the
mouth, locate the thick-walled pharynx. The esophagus extends from
the pharynx. Next, 2 swollen structures can be seen, the crop and
the gizzard. The crop temporarily stores food and the gizzard then
grinds it. Leading from the gizzard is the intestine, which runs the
length of the worm to its anus. The earthworm feeds on organic
material in soil, pushing this material through its digestive tract
and absorbing nutrients.
Locate the dorsal blood vessel, which is found along the dorsal
surface of the digestive tract. Identify the 5 pairs of aortic
arches, or hearts, which circle the esophagus.
Identify the cerebral ganglia, which are found along the dorsal
surface of the pharynx. A ventral nerve cord can be seen beginning
at the cerebral ganglia and extending the length of the worm.
Locate the excretory organs called nephridia. These paired
organs are found in each segment. Nephridia remove nitrogenous
waste.
Earthworms are hermaphroditic, meaning they have both male and
female sex organs. Eggs are produced in the ovaries, which can be
found in segment 13. Eggs pass through the female genital pores.
Sperm are produced in the testes, found in segments 9 to 12. Sperm
pass through the male genital pores. Study the segments containing
the gonads and try to identify each sex organ.
During mating, sperm from one worm travel to the seminal
receptacles of another worm. Once the 2 worms have separated,
fertilization of the eggs occurs in the cocoon created by the
clitellum. First the cocoon is moved over the outside body wall
where eggs are deposited, and then it is moved over the seminal
receptacles where sperm are deposited. Fertilization occurs outside
the body.
Once you have observed the structures of the earthworm, dispose
of the specimen in accordance with local guidelines and your
teacher’s instructions.
