Earthworm: Anatomy And Dissection (1961)

[Music] One way we learn about animals and plants is by dissection. We'll need a dissecting tray filled with wax, some pins, a needle, tweezers, scissors, and a scalpel or knife. Dissection will enable us to observe in detail the structure of an earthworm. Earthworms are common and familiar representatives of the phylm anelada. These worms have been preserved for dissection. We'll select one for our specimen. The body is made up of as many as 100 to 175 segments. The front or anterior end is more pointed than the posterior end. There is no visible separate head. On the anterior or front end is the mouth. On the posterior end is the opening of the intestine, the anus, a vertical slit in the worm. On the under or vententral side are small openings. These are the seinal vesicles which store the sperm cells. On two of the segments are the seinal receptacles which receive the sperm. About 30 segments back from the anterior end is a thickened band called the clellum. Later we'll see how this functions in sexual reproduction. We are now ready to begin the first step in the dissection. In the worm, we can remove the posterior region since it includes no organs essential to our dissection. Removal of this portion gives us a cross-section of a segment which we can see in this closer cross-sectional view. In each segment, there are four pairs of CT or bristles embedded in the skin. When extended, the CT enable the worm to gain traction. We are now ready for the next step in our dissection. With the worm top or dorsal side up, we use the scissors to cut the skin. Great caution must be taken not to cut too deeply so that we do not rupture the intestine. We now work with extreme care since we do not want to damage the vital organs which lie in this region between the clutellum and the head. Now we are ready to expose the organs. First we temporarily anchor the worm to the wax surface by placing one pin at this point. The knife which must be razor sharp is brought just under the skin very gently to free the membranes. As the skin becomes free on both sides, we place in pins, not tightly but loosely to prevent the skin that hasn't been freed from pulling and breaking and damaging the specimen. After we have made the cut, we can tighten all of the pins. The first system we will examine is the digestive system. At the anterior end of the body is the mouth which has no jaws or teeth. It opens into the fernx which is a muscular tube used to suck in material containing food. Directly in back of the fairings is the esophagus through which food particles pass. The esophagus opens into the crop where the food is temporarily stored. The crop leads to the gizzard where the food is ground up emptying into the intestine where digestion is completed. The earthworm consumes soil which contains organic matter. Useless material passes through the intestine and is deposited in the earth. Now let's review the digestive system using this simple diagram. Here are the mouth, fernx, and esophagus. Here are the same organs in the specimen. mouth, fernx, esophagus. The drawing shows how the esophagus opens into the crop which leads to the gizzard. From here, food empties into the intestine. This is the crop, gizzard, and intestine. Now let's examine a second system, the circulatory system. As food is digested by the intestine, blood picks up the digested food for distribution to all cells in the body. The blood flows forward to the anterior end in the dorsal blood vessel and moves toward the posterior end through a vententral blood vessel. This vessel can be seen by lifting a portion of the intestine. Small tubes connect the dorsal and vententral vessels throughout the system except in the region of the five aortic arches. These muscular tubes function as hearts. On a live worm, we see the aortic arches keeping the blood flowing by alternate contraction and relaxation. The major organs in the circulatory system are the aortic arches which you remember function as hearts, the dorsal blood vessel and the vententral blood vessel. Small tubes connect the dorsal and vententral vessels except as we saw in the aortic arches which are enlarged tubes. A third system the excratory system consists of a pair of coiled tubes called nefridia in each segment which we see here in cross-section. Two nefridia are present in each segment and function as kidneys. The nephridia remove waste materials and transport them outside of the body. A fourth system is the nervous system. Here is the brain. Here are the two nerves which join and form the long vententral nerve cords. Here are the ganglia which are present in each segment. The needle at the left will point to the brain and the two nerves which join and form the vententral nerve. Here is a photo microraph of a portion of the vententral nerve cord showing several ganglia which are present in every segment. The earthworm has no eyes or ears but is sensitive to light and vibration. Now let's view the organs which make up the reproductive system. The reproductive system, the fifth system, can be illustrated with two earthworms. Earthworms are hermaphroditic animals. This means that each worm can produce both eggs and sperm. Let's view a diagram of two worms exchanging sperm. The sperm travel from the opening of the seminal vesicles of this worm to the seinal receptacles of the other worm. In a similar manner, sperm travel from this worm to the other. This exchange occurs in a groove within the band or tube formed by a sticky substance secreted by each clutellum. After the worms separate, the fertilized eggs are gathered into cocoons. The cocoons are left in the earth where the young develop. Here we see the seinal vesicles which store the sperm. Now that we have seen and discussed the major organs of the earthworm, let's view a cross-section so that we can see the components of one segment. This is the dorsal blood vessel. Through the center is the intestine surrounded by the body cavity. The vententral blood vessel is just below the intestine. The vententral nerve cord is below the vententral blood vessel. There is a nephridium on each side of a segment. These are the CT or bristles. When you perform an earthworm dissection, you will observe the principal organs and how they function as we have been doing in this dissection of an earthworm.