Biology-Mr.Willets
Lab-Earthworm Dissection
The common earthworm, Lumbricus terrestris, is a typical member of Phylum Annelida. Members of this phylum have a true coelomic cavity with the internal organs attached to the body walls. They have blood in a closed circulatory system. This means that the blood remains in the blood vessels. Their gut has many specialized organs. Oxygen diffuses into their body through the moist skin. They have a dorsal brain and ventral nerve. This is the first phylum of what are called the higher invertebrates.
EXTERNAL ANATOMY- Locate the following structures. Use the hand lenses to see certain structures.
a) Notice the lines on the earthworm's skin. These are the places on the inside where the internal organs are attached.
b) Anterior end- it is larger and darker than the posterior. The coloring is due to blood vessels in this area.
c) Dorsal surface- a dark line runs down the dorsal surface. This is the dorsal blood vessel (clever name)
d) Ventral surface- generally flatter and lighter than the dorsal surface.
e) Mouth- opening at the anterior end. It is covered by a "lip" called the prostomium.
f) Anus- a slit-like opening at the posterior end.
g) Clitellum- a swelling at about segments #32-37. This produces a slimy material that holds the worms together when sperm is exchanged. It also produces this material to form a cocoon that the fertilized eggs develop in. In other words, its function is reproductive.
h) Setae-(sing-seta) these are small bristles that extend from the ventral surface. You can feel these by rubbing your hand along the ventral surface. They are used for "gripping" the ground as the worm moves.
i) Sperm duct openings- On segment #15, there are a pair of openings through which sperm leave the worm. There are other openings for the fertilized eggs to leave that are too small to be seen.
j) Sperm grooves- there are small grooves on the ventral surface from segment #15 on back to the clitellum. Sperm travel along these to get from one worm to the other during mating.
INTERNAL ANATOMY- Read this entire section before cutting
When starting the dissection, some people like to pin the worm down at both ends. I do not recommend this. If you do, be careful with the pin at the anterior end. Put it through the prostomium or segment #1 not through segments #2 or #3 (otherwise, you will damage the brain).
Make sure you know which side is dorsal. Your cut must be made through the dorsal surface. You may practice on the posterior part of the worm. The important part is from the clitellum forward. This is the only part that has internal organs to identify.
Use the scalpel to make a cut through the skin of the dorsal surface. Do not cut too deeply. Make this first cut about two inches long to start. When this first cut is complete, you must carefully cut the attachments that hold the internal organs in place. These pieces of tissue are called septa. Use the sharp probe to reach in and cut the septa. Do this on both sides. When all the septa in the 2 inch section have been cut, you may pin back the walls. Continue cutting with the scalpel stopping every inch or so to cut the septa and pin the walls down. Do this until the worm is completely open.
You may now identify the structures as listed on the back.
REPRODUCTIVE SYSTEM
The earthworm is hermaphroditic with cross-fertilization. It has testes, ovaries and sacs to store its own sperm and sacs to store the sperm from the worm with which it mates.
The cream colored structures in segments #9-12 are sperm storage sacs. The testes are attached to these. The ovaries are in segment #13. They are also cream colored.
DIGESTIVE SYSTEM
The earthworm eats dirt and digests any organic matter in it. The indigestible material passes out the anus. The digestive system of the earthworm has several specialized organs. From the mouth backward, they are:
a) Pharynx- this is a muscular tube that pushes the food down through the rest of the system. The muscles fibers on it make it look a little "hairy"
b) Esophagus-This really can't be seen since it is covered by the reproductive parts (see above) and by the "hearts" of the worm (see below). Chemicals are added to the food in here to neutralize acid in the soil. This is one reason why earthworms are good for the soil. Their feces make the soil less acidic. They also loosen the soil as they move through it. You should be able to point out the area the esophagus is in.
c) Crop- this is a thin walled food storage sack. It is soft when touched with the probe.
d) Gizzard- This is a muscular food grinding organ. This is easily identified because it feels hard.
e) Intestines- these extend from the gizzard all the way to the anus. Food is digested in the intestines. A blood vessel can be seen on top of the intestines. If the intestines has been cut open, you will see the dirt that the earthworm ate.
Go back and identify the digestive organs again-just to be sure! You might start with the gizzard (since it is the most easily identified) and work forward and backward from there.
CIRCULATORY SYSTEM
The earthworm has a closed circulatory system. The blood remains in the blood vessels. The following parts can easily be seen:
a) Dorsal blood vessel- this was visible through the skin of the worm. It runs along the top of the intestines.
b) Aortic Arches- these are 5 blood vessels that act as the earthworm's "hearts". They are on the sides of the esophagus in segments #7-11. They appear as dark objects. You may have to poke around and move the reproductive parts to find all 5.
NERVOUS SYSTEM
Annelids have a dorsal brain and a ventral nerve cord. This is called the annelid type nervous system (other groups also have this type of nervous system but it named for the annelids).
a) The brain is located on top of the pharynx in segments 2-3. It appears as two light colored dots. They are smaller than the head of a pin. You may have to poke around in the muscle fibers to see it.
b) The ventral nerve cord can be seen underneath the intestines as a light colored line. It is easier to see this if you remove a section of the intestines. You may notice that it has enlargements in each segment- An enlarged section of a nerve is called a ganglion. These act as "mini-brains". If you expose the nerve nicely, hand lenses can be used to see the ganglia more clearly. The dorsal brain connects to the ventral nerve cord by two connective nerves that go around each side of the pharynx. These will be tough to find.
Remember- coelomic cavity, closed circulation, complex digestive system, dorsal brain/ventral nerve.