Introduction to Nematoda

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Introduction to Nematoda

You will study the reproductive system of Ascaris sp. Males and females live in the intestines of man and pigs where they graze on the intestinal contents.  Eggs pass out with the feces and if they contaminate food, are introduced to another host.  The larvae hatch in the intestine of the new host and then burrow through the walls to be carried by the bloodstream to the lungs.  At the lungs, they burrow through the alveoli (air sacs) and crawl up the trachea and down the esophagus.  Occasionally larvae get lost and crawl up the esophagus and exit the nose.  In some areas of the world, Ascaris is so common that a child is not considered part of the tribe until a larva is sneezed out and found in the bed!

Nematode Dissection

You will dissect Ascaris sp. to observe the internal features of the digestive and reproductive systems.

1. Before beginning your dissection, add a little bit of water into the dissecting pan. This allows the organs to float and makes internal structures easier to see.

2. Pin the anterior and posterior ends of the animal to the bottom of the pan and carefully slit the body wall longitudinally. 

3. Pin the sides of the wall to expose the internal contents.  Place pins at a 45 angle to the pan.  Be very careful not to remove or disturb any organs you will need to identify. 

4. Locate the muscular pharynx, just behind the mouth. The intestine will appear as a thin ribbon-like structure that extends from the mouth to the anus (see Figure 12).  All other internal structures are related to reproduction!

Question 1: How does the structure of the Ascaris intestines reflect the diet of this organism?

Ascaris are dioecious animals (i.e. have separate sexes).  Larger animals with the straight tails are females.  The thinner animals with hooked tails are males (see Figure 12). When you are finished with your dissection, be sure to trade with a group that dissected an animal of the opposite sex.

**CAUTION:  Ascaris eggs are extremely resistant to chemical treatment.  Although it is unlikely, some eggs may survive immersion in preservatives for short periods.  For this reason you should keep your hands away from your mouth and nose while performing this dissection and wash your hands afterward.**

Female Ascaris

1. Locate the genital pore (a slit-like opening about one third of the way down from the anterior end and connected to a short vagina).  The vagina divides into two uteri.  At the end of each uterine segment, a thin oviduct can be seen, followed by the thread-like ovaries.  Eggs are produced by meiosis within the ovaries and move along the oviducts to the uterus.  Fertilization of the eggs takes place in the uterus.

Female Ascaris sp. dissection.

Question 2: What is the purpose of the copulatory spicule on the male Ascaris? Explain the full reproductive cycle of this organism as it infests a human being.


Figure 12: Internal anatomy of  Ascaris sp. female (a) and male (b). 

Male Ascaris

1. The anus serves both an excretory and a reproduction function.  The copulatory spicule (a hook-like appendage used to hold the female’s genital pore open; they are difficult to find) should be seen at the edge of the anus. 

2. At the posterior end of the intestine, you will the cloaca (which is connected to the anus on the outside).  The cloaca serves as a common collecting area for fecal material from the intestine and spermatozoa from the seminal vesicle (the thick organ connected to the intestine at the cloaca).  The relatively thick tubes associated with the seminal vesicle are the sperm ducts or vas deferens.  The finest of the threads are the testes.  Sperm are produced in the testes and they mature as they move along the vas deferens to the seminal vesicle.  During copulation, the sperm enter the cloaca before being deposited in the female.  Nematode sperm are unusual because they are amoeboid, not flagellated. 

Question 3: Why is the reproductive system of Ascaris so extensive?

Male and female Ascaris sp.

Figure 13:  Diagram of Ascaris sp. cross section .

As seen on the cross section, the musculature of  Ascaris sp. consits of only longitudinal muscles. Groups of these muscles contract simultaneously working against the fluid in the pseudocoelom.

Question 4: Explain the musculature of the nematode. What would be an advantage to having muscles that run circular as well?


  1. Find the nematode anatomy page to your worm packets. The bottom image is the male and female ascaris. Make sure you can identify each of the structures listed on this sheet in your dissection, these will show up on the test.

  1. Color code your diagram so each part is colored a different color.

  1. Next to the name of each structure, give a brief definition of the function. The functions can be found in the dissection reading.

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