Date______________________________
Introduction:
Dissection is a process to help one understand the form and function of an organism. The main objective of any dissection is to expose organs and not to destroy them. It is one thing to view an organism's internal organs in a textbook and to actually see them up close in a properly performed dissection.
Materials:
1. Hold the worm and rub your fingers along its lateral surfaces. What do you feel? ________________
_____________________________. What are these structures called? ____________. What is their
function? _________________________________________________________________________
2. Place the preserved worm ventral side down on the dissecting tray. Make sure the dorsal side is facing you. The dorsal surface should be darker in color than the ventral side.
3. Pin the worm to the tray at the anterior and posterior ends. Measure the length of the worm in cm. _________________. Count the number of segments on your specimen. __________________.
4. Label the external features of the earthworm.
5. Measure 5 cm. behind the clitellum; make a small horizontal cut in the body wall. This will allow you to place the scissor's point into the body cavity (coelom) . Now cut upward and forward ( anterior), until you reach the end of the worm.
6. You will notice that the body wall of the worm will not move down into a flattened position. It is being held in position by small internal walls called septa. These septa must be cut in order for the body wall to lay flat. Use your probe to break these septa.
7. Now pin down the body wall. Make sure you use as few pins as possible. This will allow you to see the internal organs clearly.
8. Locate the tube like digestive system. Identify the following parts and label them on figure.1 at the end of the lab. Mouth, pharynx, esophagus, crop, gizzard, and intestine.
9. Examine the dorsal surface of the digestive system. Notice a black line running down its entire length. This is the dorsal blood vessel. Label it on fig.1.
10. The earthworm contains _________ aortic arches. Locate them surrounding the esophagus. Label them on fig.1.
11. With your probe, push aside the intestine and locate the ventral blood vessel and nerve cord. label them on fig.1.
12. Identify the earthworm's reproductive structures, located along side the esophagus. The first set of lightly colored organs are the seminal receptacles. What is their function? __________________________
____________________________________. Posterior to them are the larger seminal vesicles. They
function in __________________________________________________________. Label these
reproductive organs on fig.1.
13. The earthworm removes its nitrogenous waaste through small tube-like structures called nephridia. Each segment contains 2 nepherida, one on each side of the alimentary canal. Located the nephridia on figure 1 and label them.
fig.1
Summary:
1. How many segments does your specimen contain? ____________. How many setae does it contain? ____________.
2. How did you arrive at the above answer? _______________________________________________
__________________________________________________________________________________.
3. Describe the form of the crop and gizzard. _______________________________________________
__________________________________________________________________________________
4. How does their form relate to their function? _____________________________________________
___________________________________________________________________________________
5. What is a hermaphrodite? ____________________________________________________________
__________________________________________________________________________________
6. How does the earthworm remove nitrogenous waste from its body? ___________________________
__________________________________________________________________________________
7. Explain why the earthworm's circulatory system is considered a closed one?
__________________________________________________________________________________
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