Biology-Mr.Willets
Lab- Ascaris Dissection

Phylum Nematoda are the roundworms. Roundworms are found everywhere: land, fresh-water, salt water and inside hosts as parasites. Ascaris is a roundworm parasite that lives in the intestines or liver of many mammals including humans. Ascarid type worms are very common in dogs. Because it is a parasite, it produces huge numbers of offspring and you will notice that most of its internal organs are reproductive.

When you open the worm, you will also notice that there is a body cavity (space) in which the organs are located. It will be obvious that the internal organs are not attached to the walls of this cavity. This tells us that it is a pseudocoelomic cavity. Roundworms have two openings to their gut- a mouth and anus. This is called a complete gut.

Examine your worm. It is a female. A male and female can be compared on the back. Notice that the skin is smooth and translucent. The anterior end can be identified by the presence of three rounded lips (see diagram on back) Use the hand lens to see them. Remember which end is anterior.

On the outside of the worm, there are 4 lines that you might be able to see that run from one end to the other. Two of these are nerves (ladder type nervous system) and two are excretory tubes. Do not spend a great deal of time searching for these.

The skin of the worm can be easily slit open with the sharp probe. Insert the probe through the skin at the posterior end of the worm. Do not insert too deeply. Run the probe up the length of the worm to open it. When completely slit open, use the pins to hold the body wall flat to the tray. Put the pins in at a 45° angle so that they do not get in the way of your examination. Use as few pins as possible.

When the body wall is completely pinned back, add a little water to the worm. This makes it easier to move the organs around. You can see now that the internal organs can move freely within the cavity.

Structures to Identify-Use the diagram on the back to help GO TO DIAGRAM

I. Reproductive System- this takes up most of the space in the body cavity. It is made of 4 major parts.

Read parts a-d below and consult the diagram on the back before you start poking around.

a. Ovaries- these are the smallest of the coiled tubes in the body cavity. There are two of them but they are tangled so that they do not appear as separate structures.

b. Oviducts- the ovaries enlarge to form the oviducts. These are the medium size tubes.

c. Uterus- (pl-uteri)- Each oviduct attaches to a uterus. These are the very large tubes. They have a 'bumpy' look to them. There are two uteri lying next to each other. The uterus is where the fertilized eggs are stored prior to being released.

d. Vagina- near the anterior end, the two uteri join in a Y-shaped connection to form a single tube-the vagina. This connects to the outside through a genital pore. Sperm enters through this pore and the fertilized eggs leave through it. You might have ripped this when you opened the worm.

II. Intestines- a flat, ribbon-like structure that runs the entire length of the worm. You might have damaged parts of it when opening the worm. You should still be able to find parts of it intact. Since this is an intestinal parasite, the digestive system is not well developed.

III. Examine the inside of the body wall. The two nerves and two excretory tubes might be more easily seen now. Rub the probe along the wall to see how flimsy the muscles are. This is typical of parasites which do not have to move very much.

When you have identified these structures, call me over to check. Remember, the three most interesting features of this worm: A) pseudocoelomic cavity B) large reproductive system C) complete gut