INTRODUCTION: If numbers alone are used as a measure of success, the jointedlegged animals are the most successful animals. Their body segmentation suggests that
they evolved from segmented worms. The segments, however, are more specialized, and
many segments have an appendage.
Sense organs, such as eyes and antennae, muscles arranged in groups, jointed
legs, and a protective body covering have made it possible for these animals to become
more successful than any other group of animals. Although jointed-legged animals vary
greatly in structure, the basic body plan is the same.
The crayfish is a large aquatic arthropod, which means it carries its skeleton on
the outside. Because of its size, and representative anatomy of the jointed-legged
animals, it is a useful dissection specimen. Please use this experience to study the
general organization of the external anatomy of this group of animals.
PURPOSE: To examine the crayfish as a representative example of the jointed-legged
animals and to become familiar with its external and internal structures.
STANDARDS:
1.4 Write in various modes in all content areas
1.5 Produce writing of high quality
3.1. C Explain the parts of a simple system and their relationship to each other.
3.3.A Describe the similarities and differences that characterize diverse living things.
EXTERNAL ANATOMY
PROCEDURE AND OBSERVATIONS:
A. Put on goggles, a lab apron and gloves. Place the crayfish in the dissecting tray with
its top side up. Like all crustaceans, a crayfish has an exoskeleton that covers its
body. Look at the exoskeleton and find the two sections, the cephalothorax and the
abdomen. The part of the exoskeleton that covers the cephalothorax is called the
carapace. The abdomen is behind the cephalothorax and is also divided into sections.
Count them. The abdomen ends in a segment called the telson.
1. How many segments does the crayfish have in its abdomen?
B. The carapace extends toward the front of the animal’s body and forms a hard beak
called the rostrum. Find the compound eyes that lie beneath the rostrum. Examine
the eyes with a hand lens and see if you can observe the many lens facets that make
up a compound eye. Observe the diagrams and models of lens facets/compound eyes
provided by your teacher.
2. What is the difference between a compound eye and your eye?
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C. The appendages closest to the front of the animal’s body are branched antennules.
Locate the large antennae just behind antennules. The antennules are organs used for
balance touch and taste. Long antennae are organs used for touch, taste, and smell.
3. What is the difference in structure of the antennae and antennules?
4. How many antennae does a crayfish have?
5. What is the difference in function of the antennae and antennules?
D. Turn the crayfish over and examine the structures on the bottom of the animal’s body.
Locate the mouth and the mandibles (jaws). Try to move the mandibles and see how
they move in relationship to the crayfish’s mouth/face. The mandibles, or jaws, crush
food by moving from side to side.
6. Describe the structure of the mandibles.
7. In which direction do the mandibles move?
8. How is this different from the way your jaws move?
E. Look near the mouth and find the three pairs of mouthparts known as maxillipeds.
These are used during eating. Two pairs of maxillipeds hold solid food, tear it, and
pass it to the mouth. The third pair of maxillipeds draws water over the gills.
9. How does the shape of the maxillipeds differ from the walking legs?
F. The next pair of appendages is the most obvious. These are claws. The chelipeds are
used for defense and to capture prey.
10.How is the cheliped shaped to fit its function?
G. The next four pairs of legs are called walking legs. Examine them carefully.
11.How do the walking legs differ in shape?
12.What are the functions of the appendages that are found on the
cephalothorax? (all of the ones you have already observed)
H. Now, observe the appendages on the abdomen. We can use these appendages to
determine the sex of your crayfish. The first five segments of the abdomen each
have a pair of swimmerets, which create water currents and function in reproduction.
All of the swimmerets in the female are small and soft. In the male, the first two pairs
are hard and modified for transferring sperm. Now, find the place where the walking
legs attach to the body. If you think you have a female, you can use a magnifying
glass to find the crescent shaped slit that is the genital pore near where the 3rd pair of
walking legs attaches. In a male, the sperm duct openings are found where fourth
pair of walking legs attaches. After determining the sex of your specimen, be sure
you have observed the opposite sex.
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13.What is the sex of your crayfish?
14.How did you determine the sex of your crayfish (what did it possess)?
I. At the back end of the animal’s abdomen is a pair of uropods, which together with the
telson form the powerful tail fin, used in backward swimming. The crayfish moves
backward by forcing water forward with this structure. Please note that the telson is
an extension of the last abdominal segment, and not an appendage. Locate the anal
opening on the bottom side of the telson.
J. Now that you have examined the external anatomy of your crayfish, you will study
the appendages, which the crayfish possesses. Observe all the appendages and
compare them to the diagram attached to your lab. Be sure to look at the shape
(structure) and relate it to the function of that appendage. Remember that the telson is
not an appendage!
15. How did you see that form fits function in the crayfish appendages?
This is the end of the first day of the dissection. When you have reached this
point, clean up your area thoroughly and work on the questions for this portion
of the dissection. Do not go any further!
INTERNAL ANATOMY
PROCEDURE AND OBSERVATIONS:
A. Put on glove, a lab apron, and safety glasses. Place the crayfish in the dissecting pan
dorsal (top) side up. Carefully insert the point of the scissors under the top of the
shell at the point indicated by your instructor and cut up the midline to the rostrum.
Cut across the carapace just behind the eyes and remove the two pieces of the
carapace. Be careful to remove the carapace slowly or you will rip some of the
underlying tissue and may lose some organs that you must observe. Note the exposed
gills. Each gill has blood vessels running through it and is feathery in nature,
providing a large surface area for absorbing larger amounts of oxygen from the water.
Move the walking legs and notice what happens to the gills.
16. Why are the gills branched?
17. To which structures are the gills attached?
18. What is the advantage of the gills being attached to the legs?
B. Remove the gills from one side and carefully separate the muscles in the thorax to
locate the light-colored heart just below. The crayfish has an open circulatory system
in which the blood flows from arteries into sinuses or spaces in tissues. There are no
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veins. The blood flows over the gills before returning to the heart. Remove the heart
and cut out the sides of the thorax to expose the organs underneath. The two lightcolored masses extending on each side of the body into the head are digestive glands.
They produce an acid that is needed in the intestines to chemically break down food
the crayfish eats.
19. How do the digestive glands help in digestion?
20. Why is it good to have the digestive glands attached to the intestine?
C. Between these digestive glands you should see a small pair of white reproductive
organs in the male animal. The female will probably have a large mass of darkcolored eggs. These eggs must be removed before proceeding. To locate the
intestine, insert the point of the scissors under the topside of the shell and cut back to
the telson. Spread the shell, and the intestine will be found as a tube on the topside of
the muscles of the abdomen. Trace it forward to the point where the intestine joins
the large, thin-walled stomach. Cut into the stomach and examine the interior walls.
Locate the hard, tooth-like structures which function in mechanical digestion of the
food the crayfish eats. What does this mean? Where does this happen in your body?
Your crayfish may also have food remaining in its stomach!
21. How do the hard, tooth-like structures aid the stomach in digestion?
D. Now, remove all of the organs by cutting the short esophagus below the stomach and
the bands of muscle holding the stomach just behind the eyes. You should be able to
lift out most of the internal organs in one piece. Clean out the remaining tissue in the
head so that the green glands are exposed just behind the antennules. These
structures function like your kidneys. What does this mean that they do for the
crayfish?
22. What color are the green glands? (They are NOT green.)
23. What is the function of the green glands?
E. In the front part of the head cavity, between the eyes is a small mass of white tissue,
the brain. Trace the nerves that go from the antennae and the eyes. Then trace the
nerve cord back from the brain to the abdomen. The enlargements of the nerve cord
in each segment of the abdomen are the ganglia. You should be able to find the nerve
cord that runs on the ventral (bottom) side of the crayfish. The crayfish uses its
abdomen (tail) for defense and can curl up or use the tail to quickly swim away if
threatened.
24.Why is the crayfish’s nerve cord on the bottom of its body?
.
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Name: ________________________________________ Period: ______ Due Date: _________
You will need to choose 3 assignments to complete from the Tic-Tac-Toe board below.
You must choose the middle block and you should make Tic-Tac-Toe with your other
choices if possible. Please indicate the choices you have made by placing your initials in
your 3 boxes on the board below and attach your work, or proof of your work when you
submit your crayfish materials. **If you have another idea for one of the blocks,
please see Mrs. Lock. The middle block is not, but some blocks may be negotiable.
Read the lab and make a
list of every structure you
need to label on your
diagrams. Write the
function (job) of each next
to its name. (attach your
list to your packet)
Read the lab and make
flashcards for each
structure you need to
label on your diagrams.
The name should go on
the front and the function
should go on the back.
(attach your flashcards to
your packet)
Read the lab and draw
sketches of the structures
you will need to label on
your diagrams. Write the
name of each structure
beneath the sketch. (use
the descriptions, your text
and the materials on your
tables to help you)
Read the lab and highlight
every structure you need
to label on your diagrams.
Underline the function of
each structure.
Complete the virtual or
physical crayfish
dissection. Answer the
questions, label the
diagrams and write the
conclusion.
Write a short poem or
song that follows the path
of food through a crayfish
from mouth to anus.
Obtain an article about
crayfish or some other
arthropod from Mrs. Lock.
Create a crossword puzzle
Read the article and
about the crayfish using
summarize it in one
the structures you labeled
paragraph. Then, in a
as the answers and their
second paragraph,
functions as the clues.
describe the adaptations
that arthropod has to be
successful.
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Use a good science
website to find an article
about an arthropod.
Summarize the contents
of the article in a
paragraph, or make your
own illustration/picture
for this article.
Please answer the following questions in
complete sentences.
EXTERNAL ANATOMY:
1.
How many segments does the crayfish have in its abdomen?(don’t count the
2.
What is the structural difference between a compound eye and your eye?
3.
What is the difference in structure of the antennae and antennules?
4.
How many antennae does the crayfish have?
5.
What is the difference between the function of the antennae and antennules?
6.
Describe the structure of the mandibles.
7.
In which direction do the mandibles move?
8.
How is this different from the way your jaws move?
9.
How does the shape of the maxillipeds differ from the walking legs?
telson and uropod)
10. How is the cheliped shaped to fit its function?
11. How do the walking legs differ in shape as you move from front to back?
12. What are the functions of the appendages that are found on the cephalothorax? (all of
the ones you have already observed; use a sentence or 2 to describe these)
13.
What is the sex of your crayfish?
(male or female; NOT boy or girl!)
14. How did you determine the sex of your crayfish (what structures did it possess)?
15. How did you see that form fits function in the crayfish appendages? (2 PTS)
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Crayfish External Anatomy
(Section of the body)
(Section of the body)
INTERNAL ANATOMY:
16. Why are the gills branched (think about their function)? (2 PTS)
17. To which structures are the gills attached?
18. What is the advantage of the gills being attached to the walking legs? (2 PTS)
19. How do the digestive glands help with digestion?
20. Why is it good to have the digestive glands attached to the intestine? (2 PTS)
21. How do the hard, tooth-like structures aid the stomach in digestion? (2 PTS)
22. What color are the green glands? (They are NOT green.)
23. What is the function of the green glands?
24. Why is the nerve cord on the bottom of its body? (2 PTS)
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_____________
_____________
__________________
____________
_____________
_
___________________
_____________
_____________
_____________
________________
___________________
CRAYFISH CONCLUSION: (10 PTS)
You have just finished examining the internal and external crayfish anatomy. Looking
back on this dissection and observation, please write a 3-paragraph conclusion on a
separate sheet of paper using the following outline:
 Paragraph 1:
 What is an invertebrate
 What is an arthropod
 Thesis/topic sentence that tells why we are dissection crayfish (think
about the first 2 parts of his paragraph)
 Paragraph 2:
 Symmetry
 Organs
 Digestion
 Movement
 Paragraph 3:
 How does “Form fit function” in the crayfish?
 Why is the arthropod considered advanced?
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