Download Earthworm Dissection - Glassford Bio

Earthworm Dissection
Classification of Earthworm
Kingdom Animalia: Multicellular eukaryote, heterotrophic
Phylum Annelida: body made of many segments separated by a septum. Two body openings and a
closed circulatory system. Ex. Earthworms, marine worms, leeches etc.
Class Oligochaeta: lack appendages and very few bristles. The can be terrestrial or found
in fresh water
Objectives:
1. Understand all major earthworm systems for
maintaining homeostasis.
2. Correctly dissect an earthworm and accurately
identify various earthworm organs and systems.
3. Understand how the earthworm fits into the world.
4. Master (85%) both the dissection quiz (in class) and
the online guide quiz (UTIPS).
All living things must maintain a relatively stable internal environment to survive. The term used to
describe this is "homeostasis." Living things carry out different essential functions in order to accomplish this
task. Multicellular organisms have “systems” that work together to maintain this stable environment. For
example when you run your body is placed under sever stress. Various systems will start to work overtime
keeping your body alive. Your heart starts beating faster and your breathing increases. Your body is trying
to deliver oxygen and take away carbon dioxide (respiratory waste) form working muscles etc. Your skin
starts to sweat reducing the excess body heat created by the activity. Without your body systems working
together your systems will fail and death would ensue. There are many more system accommodations that
occur as you run or engage your body in different activities.
Multicellular organisms maintain homeostasis differently. Compare the differences between warmblooded
and cold blooded organisms. To adjust body temperature insects or reptiles will move their body to a
location that will ether warm or cool their body depending on their needs.
Below are a few examples of various systems found in multicellular organisms.
Digestive /Feeding
Response /Nervous
Movement
Circulatory
Respiratory
Reproductive
Excretion
Single celled organisms do not have systems because they are only made of one cell. Thus each cell must
carry out all of these functions without relying on tissues, organs and organ systems (ex. bacteria, yeast,
and most protists).
Feeding (digestive), Earthworms have a fleshy projection on the anterior end called the
prostomium. This structure covers the mouth. The mouth leads into a muscular pharynx that
pumps food into a storage organ called the crop. Just posterior to the crop is a thick-walled, highly
muscularized gizzard. Small rocks (grit) in the gizzard help to grind food before it is moves into the
intestine where the nutrients from the food is absorbed. After absorption solid waste is eliminated
through the anus. The solid waste of earthworms are called castings. Worm castings are highly
sought after because of their concentrations of nitrogen, phosphorus, potassium, micronutrients and
beneficial bacteria. Worm feces is a fantastic fertilizer and very expensive. Earthworms also help to
till / mix the soil as they burrow through the ground. As a result they leave a network of tiny holes
and spaces that help water and air get to plant roots. These spaces are critical niches for bacteria to
colonize and decompose organic debris as well as fix nitrogen (nitrogen fixation).
The intestine extends from the posterior end of the gizzard and continues until it reaches the anus.
Respiration, (respiratory system)
Earthworms secrete a thin surface film of mucus to keep them
moist so oxygen and carbon dioxide can diffuse through the body
wall. Worms do not have lungs or gills like some organisms.
Remember that living things accomplish each of the tasks
necessary to maintain homeostasis differently. All living things
MUST have a way to deal with these requirements. It is your
quest to make sure you can describe and explain how different
living things meet these requirements.
Circulation, (circulatory) Earthworms have five simple hearts located just anterior to the crop
and posterior to the pharynx. These are very simple hearts / vessels that pump blood
throughout the earthworm. Earthworm blood is red because it has hemoglobin. Remember that
hemoglobin is a protein that binds oxygen. There are two relatively large blood vessels that run
along the top (dorsal) and along the bottom (ventral). There are small vessels that connect the
two. You will not be able to see these without a microscope. The earthworms circulatory system
is considered to be a closed system because the blood circulates within a network of vessels.
An open system has spaces or cavities filled with blood that surrounds body organs. The blood
in these cavities is picked up by vessels and circulated throughout the organism. Don’t forget
that earthworms have a closed circulatory system.
Hearts
Excretion (excretory system) Solid waste passing though the intestine is eliminated
through the posterior opening called the anus. Cellular waste is excreted in a different way.
Remember all cells excrete cellular waste (nitrogenous). When waste products
accumulate inside body tissues they become toxic and must be removed or the creature
will die. To eliminate cellular waste metanephridia remove toxins keeping the cellular
landscape free of excess garbage. The metanephridia are connected to openings called
nephridiopores. The cellular waste is excreted from these openings.
Response (nervous)
The earthworm, as well as most annelids,
have a well developed nervous system
consisting of a brain and many nerve
cords. You can see from this image that
the ventral nerve cord is connected to the
brain. See if you can see ganglia.
Movement (skeletal and muscular)
Earthworms alternately contract
their longitudinal (shortens) and
circular (elongates) muscles to
move.
Reproduction, (reproductive system) Although earthworms are
monecious, they undergo cross-fertilization during copulation. During
copulation sperm cells are released from the seminal vesicles and stored
in the seminal receptacles of the opposite earthworm. The clitellum slides
over the anterior segments and picks up eggs from the oviducts in the
14th segment and sperm as it slides over the anterior end of the worm to
form the egg cocoon.
Before You Start The Dissection
1. Watch my dissection video and go through this PowerPoint before you come
to class and participate in the dissection.
2. Listen and follow ALL rules /safety procedures taught in class.
3. You will complete this dissection with your desk partner.
4. Each student must get safety glasses from the UV box before starting the
dissection. The safety glasses must be worn throughout the entire dissection.
5. Listen carefully and pay attention to classroom dissection demonstration.
6. Student not listening or participating in a positive way will be asked to leave
the lab and complete book work to receive points for the dissection.
7. When finished with the dissection you will raise your hand and request a lab
quiz. A lab aid or one of you teachers will come and give you and your partner
an oral quiz. This is a team quiz, you both get the same score. We will point
out various anatomical structures for you to identify. You will be asked 10
different questions (100 pts)
8. When finished with your quiz clean your area as instructed and start taking
your earthworm dissection guide notes.
9. Take the earthworm guide quiz at home or at school ONE time only. You may
NOT retake quizzes. I strongly admonish you to take good notes to assist you
while taking the guide quiz. Guide quizzes are found on UTIPS. I
repeat….guide quizzes may only be taken ONE time. If you log on and exit
you will receive a zero. If you prepare well (take notes, ask questions, study
this PowerPoint and my dissection video) you will do well, if not……….
The next slides provide you with some still images of the
different parts of the earthworm. It is important that you
use these still images along with my dissection video in
your preparation.
We hope you will enjoy learning about the systems living
things use to maintain homeostasis.
Materials Needed To Perform Your Dissection
Pointer
Forceps
Earthworm
Dissecting Pins
Scalpel
Make sure you have a dissection guide on your desk.
Look up on the white board and you will see a list of
key anatomical structures you will be asked during
your dissection quiz. You may refer to this list while
being quizzed. You may not look at the dissection
guide.
You or your partner will go get your dissection bag
and carefully remove the earthworm. Secure your
specimen bag and return the bag to the appropriate
box. Double check and make sure both of you are
wearing safety glasses.
I have a limited supply of latex gloves for those
students who do not bring their own. Plan ahead
and place your gloves in your biology binder /
backpack. The biology schedule on the moodle
gives you the dates for all dissections.
Place your earthworm on the dissection tray and go over the external
anatomy.
Dorsal View
Posterior
Anterior
You can tell this is dorsal because it is darker in color and the
prostomium points downward. The dorsal blood vessel gives the
dorsal surface its color.
Ventral View
Anterior
Posterior
Ventrally the earthworm is not as dark.
External Anatomy
Clitellum
Prostomium
Mouth
Anus
Segments
You can see my pointer is on one of the lines
that separates the segments.
Seminal Receptacle
Seminal Receptacle
The probe is touching the Prostomium
The probe is in the Mouth
The setae are structures we cannot visualize on the
earthworm you are dissecting. If you rub your
fingers along the ventral surface it feels like sand
paper. This sensation is a result of the little bristle
like structures called setae. These bristles give
them traction as they move through their tunnels
and along the surface of the soil.
Once you have gone over the external anatomy you are ready to take a look
at the internal anatomy. Double check and make sure you have the worm
dorsal side facing up. Notice how the scalpel is placed against the outside
of the dissection tray. When the scalpel is not in use make sure the sharp
edge of the blade is facing the metal.
Place image of starting the proceedure
Use your forceps to hold the worm and gently
cut into the upper body wall. Be careful not to
damage the internal organs just under the body
wall. You are cutting through the muscles used
for movement etc.
Once you have cut through the body wall you can carefully begin to pin down one side.
Here we are pinning the body wall.
Continue to cut and pin as you go. Cut and pin just past the clitellum.
Angle your pins so you have access to the internal organs.
Continue to cut and pin until you have both
sides pined at an angle.
On the subsequent slides we
will be looking closely at the
internal parts of the earthworm
Look very careful and you can see two small white dots. These dots are the
cerebral ganglion or brain of the earthworm.
pharynx
seminal
receptacles
seminal vesicles
Anterior end
gizzard
Intestine
The probe is pointing to the crop.
The probe is under the esophagus
The probe is touching the dorsal blood vessel which runs
dorsally on top of the intestine.
hearts
seminal vesicles
Dorsal blood vessel
crop
gizzard
Seminal receptacles
All five hearts cannot be seen in this diagram because of tissue that
is covering some of the arches. You can see at least two and the
third you can see through the tissue.
ventral blood vessel
ventral nerve cord
Do not forget you must study both this
PowerPoint and my worm dissection
video before you come to class and
complete the earthworm dissection.
Good luck on your dissection !
Works Sited
Mr. Glassford’s Image Gallery
Biology Text: Miller and Levine