Earthworm Dissection

Earthworm Dissection

The following is a classification of a species in the earthworm family Lumbricidae. This common species is Lumbricus terrestris also known as the night crawler or dew worm.

Kingdom	Animalia
Phylum	Annelida
Class	Oligochaeta or Clitellata
Order	Megadrilacea
Family	Lumbricidae
Genus	Lumbricus
Species	terrestris

Lab Objectives:
• Describe the appearance of various organs found in the earthworm.

• Name the organs that make up various systems of the earthworm.

Background Information:
Among the most familiar invertebrate animals are the earthworms, members of the phylum Annelida. The word annelida means "ringed" and refers to a series of rings or segments that make up the bodies of the members of this phylum. Internally, septa, or dividing walls, are located between the segments. External segments are called metameres. There may be more than 100 segments in an adult worm. The clitellum is a swelling of the body found in sexually mature worms and is active in the formation of an egg capsule, or cocoon. Eggs are produced in the ovaries and pass out of the body through female genital pores. Sperm are produced in the testes and pass out through tiny male genital pores. During mating, sperm from one worm travel along the sperm grooves to the seminal receptacles of another worm. Fertilization of the eggs takes place outside the body as the cocoon moves forward over the body, picking up the eggs of one worm and the sperm of its mate.

The pumping organs of the circulatory system are five aortic arches. Circulatory fluids travel from the arches through the ventral blood vessel to capillary beds in the body. The fluids then collect in the dorsal blood vessel and reenter the aortic arches. The earthworm takes in a mixture of soil and organic matter through its mouth, which is the beginning of the digestive tract. The mixture enters the pharynx, which is located in segments 1–6. The pharynx is a muscular organ that pushes the food into the next portion of the digestive system, the esophagus. The esophagus, in segments 6–13, acts as a passageway between the pharynx and the crop. The crop stores food temporarily before it moves on to the gizzard. The gizzard is a muscular organ that grinds up the food by muscular contractions similar to how your stomach works. The mixture that the earthworm ingests is ground up in the gizzard, then it is moved into the intestine, which is the main organ of digestion. In the intestine, which extends over two-thirds of the body length, digestion is completed and absorption of the nutrients into the bloodstream takes place. Soil particles and undigested organic matter pass out of the worm through the rectum and anus.

The nervous system consists of the ventral nerve cord, which travels the length of the worm on the ventral side, and a series of ganglia, which are masses of tissue containing many nerve cells. The nerve collar surrounds the pharynx and consists of ganglia above and below the pharynx. Nervous impulses are responsible for movement and responses to stimuli. Each segment contains an enlargement, or ganglion, along the ventral nerve cord. Excretory functions are carried on by nephridia, which are found in pairs in each body segment. They appear as tiny white fibers on the dorsal body wall. The earthworm has no gills or lungs. Gases are exchanged between the circulatory system and the environment through the moist skin by a process of diffusion.