Crayfish External Observation          Name _______________ Period ______

Prelab Questions:

1. Crayfish belong to the Kingdom __________________, the __________________Phylum and the Class _________________,

2. List three characteristics that all arthropods share.


3. Name two other animals in the same phylum as a crayfish (related).

The Head:

Place the crayfish ventral side up so the mouthparts can be observed. (Circle each number when completed)

4. Locate the 1st, 2nd, and 3rd maxillipeds. These appendages are used for manipulating food. (The 3rd maxilliped is the largest and topmost one, the 2nd is underneath, and the 1st is underneath the 2nd) 

5. Locate the mandible which lies underneath the maxillipeds. This structure should be hard and difficult to move. The mandible of arthropods opens differently than the jaws of humans.
Describe the difference. _______________________________________________________________

6. Locate the two large antennae and the smaller antennules that branch from the base. The antennae are sense organs (touch, taste, equilibrium)

7. Locate the eyes, which extend from two stalks called pedicles.

The Body

8. The body of the crayfish consists of a fused head and thorax: the cephalothorax. The cephalothorax is covered by a thick armor called a carapace. Extending from the carapace is a pointy structure called the rostrum. Locate the cephalothorax and rostrum.

9. The abdomen of the crayfish is segmented and flexible. Bend the abdomen back and forth and observe how each segment moves.

10. Count the number of segments on the abdomen. Hint: bending it will show you were the segments are separated. How many segments are on your crayfish? ______
Compare this number to other crayfish, are they all the same? _________

The Appendages

11. Locate the chelipeds (the claws). Gently manipulate the cheliped to determine the direction in which the appendage can bend. How many joints are there on a single cheliped? _________

12. Behind the cheliped are four pairs of walking legs. How many joints are on each leg? _____

13. Locate the swimmerets (appendages attached to each segment of the abdomen). Are the swimmerets jointed? ______ How many pairs of swimmerets are there? ______

14. The last segment of the abdomen (the 7th segment) is called the telson, and it is specialized for swimming. Locate the two uropods extend from either side of the telson.

Determining the Sex of Your Crayfish

15. Look at the first pair of swimmerets on your crayfish. If these swimmerets are considerably larger and stiffer than the other swimmerets, you have a male. If the first swimmerets are about the same size as the others, your crayfish is a female. What is the sex of your crayfish? ____

16. Measure the length of your crayfish and compare it to other crayfish in the room to complete the table.

Female Crayfish (length in cm)

Male Crayfish (length in cm)

Average: Average:

Based upon your data, which sex of crayfish is the largest? _______________

17-30. Label all the parts you can see on the crayfish picture below.


CrayfishBlankDiagram.bmp (520726 bytes)


Abdomen - The abdomen is the segmented tail area. The swimmerets, telson, and uropods are attached to the abdomen.
Carapace - The protective shell (exoskeleton) of the cephalothorax.
Cephalic groove - An indentation in the carapace between the head region and the thorax region.
Cephalothorax - The combined head and thorax (this animal has no neck!). The cephalothorax contains the heart, gills, and stomach.
Cheliped - One of two big claws used for defense and food handling.
Eye - The two eyes are located towards the front.
Long antennae - Two long, sensory organs towards the front of the crayfish.
Short antennae - Two short antennae, also called antennules.
Rostrum - The area above the eyes.
Swimmerets - Five pairs of short appendages on the abdomen; they are used for swimming.
Tail fan - The telson and the four uropods. It is used for backwards swimming.
Telson - The single, hind-most extension of the tail fan.
Uropods - Two pairs of appendages on the tail fan that surround the telson.
Walking legs - Four pairs of long, jointed legs used for walking. The gills are attached to the tops of the walking legs.

Crayfish diagram and labels at
External Anatomy of the Crayfish