General Biology II – Major Life Processes in Plants and Animals
Worm Dissection
This lab activity is intended to provide an opportunity for students…
To observe some major animal characteristics
To practice a dissection
To discuss major animal characteristics
Objectives:
This lab aims to contribute to the development of the following components of the Cégep Champlain
St.Lawrence Science Program Graduate Profile:
I. Apply The Experimental Method
VI. Learn In An Autonomous Manner
XI. Develop Attitudes Appropriate For Scientific Work
XII. Apply What They Have Learned To New Situations
VII. Work As Members Of A Team
This lab also contributes to the attainment of the following elements of the 00XU objective:
1.
To analyze the relationship between structure and function in multicelled organisms
This lab also contributes to the development of the follow performance criteria of the 00XU objective:
Proper use of concepts and terminology
Accurate description of structures and their functions
Analysis of the integration of various systems in animals or plants
Justified interpretations of the structural, functional and evolutionary relationships between organs or systems
Use of techniques of observation or experimentation
Adherence to safety and environmental protection regulations
Background Information
The earthworm (Lumbricus terrestris) belongs to the Phylum Annelida (Class Oligochetae), the
marine and terrestrial worms. Numbering approximately 9000 species, the annelids fall within the
category of protostomic coelomates. They are protostomes because the mouth comes from the first
opening that develops in the embryo, the blastopore. The term ceolomate refers to the presence of a true
body cavity that surrounds the gut. This second characteristic is shared with vertebrates and
echinoderms (sea stars, sea urchins). The annelids also display the following characteristics:
(1) Cephalization -a head end. Nervous tissue and sense organs are concentrated in an anterior region.
This allows the animal to move through its environment head first as it searches for food.
(2) Metamerism – the serial repetition of similar body segments along the longitudinal axis of the body
(each segment is referred to as a metamere or somite). Metamerism is clearly visible in the earthworm.
Structures such as muscles, blood vessels, and nerves, can be repeated from one segment to the next.
Other organs such as sex organs may only be repeated within a few somites. Layers of mesodermal
epithelium (peritonea) of adjacent segments connect to form septa that divide segments from one
another.
(3) Coelomic Body Cavity – in most annelids, the coelom forms in the embryo from a split in the
mesoderm on each side of the gut. The coelom is filled with fluid and serves as a hydrostatic skeleton
(hydro=water or fluid; static=unmoving). This hydrostatic skeleton prevents the force of muscle
contractions in one location from being transferred along the entire length of the body. This ultimately
allows an efficient crawling motion of the animal through alternating contractions of strong longitudinal
and circular muscles in the body wall.
(4) Circulatory system closed and segmentally arranged.
(5) Digestive system complete and not metamerically arranged.
(6) Nervous system with a double ventral nerve cord and a pair of ganglia (concentrated bundle of
nerves) with lateral nerves in each segment; pair of dorsal cerebral ganglia.
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Materials:
worm
blunt-end probes
water in wash bottle
dissecting tray
gloves
forceps
worm dissection guide
scalpel blade and handle
dissecting pins
sharp-end probes
Safety considerations
Students must wear a lab coat.
Gloves use is required.
It is preferable to wear glasses rather than contact lenses. Students wearing contact lenses will be asked
to wear safety goggles.
The worms have been stored in formalin, a compound that necessitates a few precautions. Skin contact
with formalin must be washed immediately with soap and water. Eye or mouth contact must be
rinsed thoroughly.
Do not attempt to remove the scalpel blade from the handle. Use the scalpel blade remover to safely
discard the blade.
Discard the remaining worm and its parts in the biohazard bag provided for this purpose.
Clean your equipment and tables before leaving the lab.
Procedure I: Identify External Structures
1. First, determine the anterior (front) and posterior (rear) ends of the worm. The mouth has a fleshy
prostomium at the anterior end. The anus can be found at the more tapered posterior end. Along
the body of the worm, you will find a smooth portion that does not have the same segmented texture
as the rest. This is the clitellum, a reproductive structure that the animal sheds to form a cocoon
for developing young. The mouth is found at the end closest to the clitellum.
2. The earthworm breathes through the cuticle of its skin. For this reason the animal keeps its skin
moist at all times. It accomplishes this by secreting fluid wastes from pores (nephridiopores) that
are connected to internal structures called nephridia.
3. Next, look for the setae, hair-like bristles that project from the body of the worm. These tiny hairs
assist in movement and provide a firm hold in the ground. They are found only on the ventral side.
To orient the animal ventro-dorsally, the darker side is dorsal (back), and the lighter side is the
ventral (bottom) side.
Procedure II: Cutting into the body cavity – Identifying Internal Structures
4. Pin the worm in the dissecting tray - dorsal side up - by putting one pin through the prostomium
and another through the anal segment. Use forceps to lift the skin at a point about 5 cm behind the
clitellum and make a shallow cut with the scalpel in the dorsal side. Next, use a pair of scissors to
cut the skin towards the anus slightly to one side of the dorsal midline. Note: do not cut too deeply,
or you will damage internal structures. The nervous system is especially delicate and close to the
surface.
5. Pull back one side of the body wall with forceps and cut the septa on each side of the intestine with
a scalpel. Pin portions of the free body wall to the dissecting tray. Next, cut through the clitellum all
the way towards the prostomium with scissors. Repeat the process of severing the septa and pin the
body wall to the tray.
Keep your dissection moist with water from wash bottles. A dried-out dissection is much more
difficult to work with.
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Procedure III: Identifying Internal Structures
A. Digestive System:
6. The digestive tract of annelids is unsegmented and extends the entire length of the worm. It
includes:
a. mouth.
b. pharynx. Light color. Draws food into the mouth and passes it into the esophagus.
c. esophagus.
d. crop. Stores food temporarily.
e. gizzard. Grinds food into smaller pieces.
f. intestines. Extend from the gizzard to the anus. Digest and absorb food.
B. Circulatory System:
7. Although considered a simple animal, the circulatory system in the worm is quite complex. Look for
the following organs.
a. 5 pairs of hearts. They are dark-colored, behind the pharynx, and wrap around the
esophagus. They should be present in segments 7 through 11.
b. dorsal blood vessel. This is the main pumping organ. It looks like a dark line coming from
the hearts. The hearts are also connected to the ventral blood vessel which carries blood
from the hearts to the rest of the worm body.
C. Reproductive System:
8. The earthworm is hermaphroditic. That is, it has both male and female sex organs and two
individuals can fertilize each other when mating. Note however, that male and female sex organs
are separated and sperm and eggs do not come into contact. Therefore one individual cannot
fertilize itself. It is difficult to identify all parts of the reproductive system, but you should be able to
identify some of these:
a. seminal vesicles – store sperm – they are located near the hearts.
b. seminal receptacles – receive sperm from mating partner
c. clitellum (exterior) – fertilization takes place here where both the seminal receptacles and
oviducts are located.
d. oviducts are connected to the ovaries and eject the eggs to the inside of the clitellum for
fertilization. The ovaries and egg sacs are located ventrally.
D. Excretory System:
9. Liquid wastes in the coelom are collected through nephridia. These structures are paired and are
present in each somite except the first three and last one. The nephridia collect waste fluids from
the coelom. The nephridia are more readily identifiable in the posterior regions where there are
fewer structures.
E. Nervous System:
10. The nervous system in the worm is very difficult to dissect. Push the intestines to the side to clear
your view of the white nerve cord on the ventral side of the worm. The nerve cord has a pair of
ganglia within each segment that provide sensory and motor fibers for that segment.
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Pictures: Modern Biology, Holt
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