Comparative Anatomy Organism 3: The Crayfish

External Anatomy

Directions: Use your magnifying lens to help you examine your crayfish.

Part 1: The Cephalothorax

1. Locate the 1st, 2nd, and 3rd maxillipeds(“mouth feet”) on the ventral anterior side of your crayfish. These appendages are used for manipulating food into the crayfish’s mandible (mouth).

2.Locate the mandible underneath the maxillipeds. This structure should be hard and difficult to move. Insert your forceps in between the two parts of the mandible and observe how the mandible moves.

Check In: How is the movement of the crayfish mandible different from the movement of your jaws?

3. Locate the sensory organs on the head of the crayfish. The antennules are organs of balance, touch, and taste. Long antennae are organs for touch, taste, and smell. The eyes extend from two stalks called pedicles.

Check In: What advantage might the crayfish have by its eyes being on stalks?

4. The body of the crayfish consists of a fused head and thorax: the cephalothorax. The dorsal side of the cephalothorax is covered by a thick armor called a carapace. Extending from the carapace towards the anterior end of the crayfish is a pointy structure called the rostrum.

Diagram 1: After identifying all the body parts from the descriptions in the text, label the diagrambelow with the following terms.

Cephalothorax

Carapace

Rostrum

Antenna

Antennule

Maxilliped

Cheliped

Walking legs

Pedicles

Part 2: The Abdomen

5. The abdomen of the crayfish is segmented and flexible. Posterior to the abdomen are the uropodsand telson that form the tail fan of the crayfish.

Check In: Count the number of segments on the abdomen. ______Hint: bending it will show you where the segments are separated.

Check In: Based on the structure of the uropod and telson, predict their functions.

6. The abdomen has five pair of swimmerets (AKA pleopods). To determine the gender of your crayfish,observe the first pair of swimmerets on your crayfish. If they 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. (Look at the diagram on page 17 of your CAD to assist you.)

Check In: What is the sex of your crayfish, and what evidence supports your claim?

Internal Anatomy

You will use the scissors, forceps, angled teasing probe to assist you examing the internal anatomy. Place the crayfish with the dorsal side up.

Part 3: Cutting the exoskeleton and examining underneath the carapace

7. Carefully insert the point of the scissors under the top of the carapace (shell) at the back of the cephalothorax and cut up the middle to the rostrum (Cut 1). Then cut above the walking legs and across the carapace just back behind the eyes (Cut 2) on both sides of the crayfish. Gently remove the two pieces of the carapace with your forceps.

8.Note the featherygills just under the carapace.

Check In: What organ system are the gills part of, and what is the function of the gills?

9. Gently remove the exposed gills and walking legs by pulling each walking leg off the crayfish. Carefully separate the dorsal layer of muscles in the thorax and note the heartjust underneath.The crayfish has an open circulatory system in which the blood flows from arteries into sinuses, or spaces, in tissues. The blood flows over the gills before returning to the heart.

Check In: Is the circulatory system of the crayfish more like the circulatory system of the freshwater mussel or the earthworm?

10.To locate the intestine, insert the point of the scissors under the dorsal side of the shell of the abdomenand cut back to the telson. Spread the shell, and the intestine will be found as a dark tube on the dorsal side of the abdomen muscles. The anus is the opening that waste exits from the intestine.

11.Trace the intestine forward to where it joins the large, thin walled stomach in the front part of the cephalothorax. The esophagus connects the mouth to the stomach.

12.In the front part of the head cavity, between the eyes, note the small mass of white tissue, the brain. Trace the nerves that go from the brain to the antennae and eyes.

13. Remove a cheliped from the crayfish. Insert the point of your scissors in the joint closest to the claw and cut off a small rectangle of shell to expose a translucent “paddle” that connects the muscle tissue to the exoskeleton.

Check In: What happens when you gently pull on this “paddle”, and what connective tissue found in the human body is this similar to?

To dispose of your crayfish, wrap all of the pieces of the body and any pieces that you removed in a paper towel. Wipe off all tools and put them away in the kit correctly. Throw away the crayfish as directed by Mr. Spotts and wipe out your wax pan with another wetwipe.

Answer the following Analysis Questions in the space provided on this sheet. This lab will be due on Monday 23May, 2011.

  1. What is the major function of the exoskeleton of a crayfish? How is the exoskeleton an adaptive advantage to the crayfish?
  1. Crayfish anterior and posterior ends. Provide three pieces of evidence from your observations that the crayfish demonstrates the idea of cephalization.
  1. Write the name of the organ system that each of the following organs is a part of.

Gills ______

Heart______

Brain______

Stomach______

Carapace______

Antenna______

  1. The eyes of the crayfish extend from two stalks called pedicles. What advantage might the crayfish have by its eyes being on stalks?
  1. How does the circulatory system work together with the digestive system and respiratory system to get the crayfish the nutrients and oxygen that it needs to survive?
  1. Based on your dissections of the freshwater mussel, the earthworm and the crayfish, which organism would you say is more complex, and why?

Use evidence of cephalization, differences in the digestive systems, nervous systemscirculatory systems, presence/absence of a coelom, and presence/absence of jointed appendages (arms and legs) to support your claim.

ATTACH A SEPARATE SHEET OF PAPER TO COMPLETE QUESTION 6