BiologyName:

DoranPeriod:

Earthworm Dissection Laboratory

Introduction

Earthworms (Lumbricus terrestris) are representative animals of Phylum Annelida. An examination of external and internal structures of the earthworm will reveal major annelid characteristics.

Lumbricus is an excellent animal for study because of its body organization. Earthworms are segmented animals. Their body plan consists of many rings (anellus means “ring” in Latin). Each segment, or ring, of an earthworm is numbered from anterior to posterior. An earthworm “map” at the end of the lab lists the locations of each organ or structure by segment number.

Objectives

  • Properly use dissection equipment
  • Identify organs and structures by using the earthworm “map”
  • Label diagrams of a dissected earthworm
  • Determine major annelid characteristics

Materials (per team)

Preserved earthworm

Straight pins

Hand lens (optional)

Scalpel

Probe

Goggles

Aprons (optional)

Gloves

Dissecting Pan

Procedure

  1. Identify the anterior, posterior, dorsal, and ventral sides of the preserved earthworm.
  2. Locate the clitellum, which is a band-like structure that separates the body into two unequal lengths. The shorter section (about one-third of the total body length) is the anterior portion. The longer section is the posterior portion. The dorsal service is darker than the ventral surface and rounded. The ventral surface is lighter colored and flat. This is an example of countershading (dorsal side darker than ventral side).
  3. Please locate the external structures called setae. Your teacher will demonstrate how to locate these structures.
  4. Discuss the importance of setae with your lab partner.
  5. Stretch your earthworm out with the dorsal side up in a dissecting pan. Pin the earthworm to the pan with straight pins. Use one pin at each end of the worm.
  6. Cut through the skin and muscle of your worm with a scalpel. Make sure you start at the posterior end along the dorsal surface. CAUTION: An earthworm’s skin and muscle are extremely thin. A very shallow cut is all that is needed.
  7. Spread the edges apart by carefully cutting through thin membranes (septa) on the inside of the worm. These septa are continuous with each groove on its outer surface.
  8. Pin the skin and muscle to the dissecting pan as you spread it apart. Put the pins in at an angle.
  9. After you have completely opened your earthworm, identify the internal organs by using the earthworm “map” and the following explanations. The organs of each system are listed on the “map” beside the number of the segment in which they are located. If an organ is in more than one segment, the included segments are in brackets.

Laboratory Safety

The earthworms are preserved in a mixture of chemicals require gloves, goggles, and laboratory coats. Listen carefully to your instructor’s directions including proper exposure procedures and disposal methods.

Scalpels are extremely sharp tools that will easily cut through human tissue. It is imperative that you follow all safety procedures that your instructor teaches, reviews, and models. Make sure that if one person has the scalpel in her hand, there are no other hands near the dissecting pan.

Please circle or highlight these instructions to signify you have carefully reviewed this information. If you have any questions before, during, or after the laboratory do not hesitate to ask.

Systems Overview

Digestive System

The digestive system is a tube extending from segment 1 to the last segment. The organs, which make up the digestive system, appear as areas of either slight bulges or constrictions along the tube. Remember: Consult map for location of specific organs. Use segment numbers as a guide.

Circulatory System

A thing blood vessel extends the entire length of an earthworm. It is located above the digestive organs. It is the dorsal blood vessel. A second vessel, the ventral blood vessel, also extends the entire length of the worm. However, it is below the digestive organs and not visible unless part of the intestine is removed. The ventral blood vessel is the thinner of two strands under the intestine. A series of five aortic arches connect the dorsal and ventral blood vessels. The aortic arches surround the esophagus.

Reproductive System

The reproductive system consists of seminal vesicles, seminal receptacles, ovaries and testes, and the clitellum. The seminal vesicles are three pairs of saclike structures along the esophagus. Two very small, almost dotlike structures are the Testes and the Ovaries. Both of these structures are present in all worms, but are often difficult to observe. They are shown in the picture you will label with dotted lines. The clitellum produces a mucous slime tube during mating.

The reproductive system also includes two sets of pores visible on the exterior of the worm. Segment 14 has a pair of female pores and segment 15 has a pair of male pores. Note: The female and male pores are not drawn on any of the diagrams. Reminder: Use map to locate specific organs.

Nervous System

A ganglion mass is a small mass of white tissue in segment 3. It is usually destroyed when dissecting a worm. A white nerve cord extends along the worm’s ventral surface from segment 3 to the last segment. The ventral nerve cord can be seen by removing a short length of intestine.

Excretory System

The excretory system consists of paired organs called nephridia. These are small organs against the wall of the worm. You may need a hand lens to see them. They are present in almost every segment.

External Structures

Besides the male and female pores, other external structures are the prostomium and the setae. The prostomium is attached to the first segment and is dorsal (above) the mouth. It appears to be the first segment but is not a true segment. Setae are groups of tiny bristles, which project from most of the segments. These structures are the earthworm’s contact with the soil. They may not be visible but they can be felt by rubbing your fingers along the side of the worm.

Earthworm “Map” of Organs
Segment / Digestive
System / Circulatory System / Reproductive
System / Nervous System / Excretory
System
Prostomium
1 / Mouth
2 / Pharynx
3 / Ganglion
4 / Ventral Nerve Cord / One Pair of Nephridia Located in Each Segment
5
6
7 / Esophagus / One Aortic Arch in Each Segment
8
9 / Seminal Receptacle
10 / Testes / Seminal Vesicle
11
12 / Dorsal &
Ventral
Blood
Vessels
13 / Ovaries
14 / Female Pore
15 / Crop / Male Pore
16
17
18 / Gizzard
19
20
21 / Intestine
Clitellum
(May Extend From Segment 31 to 37)
Last Segment / Anus

Hints:

Notice this table is broken into segments (rows) and body systems (columns).

Non-shaded boxes indicate that an organ is located in that segment for a particular system.

Shaded boxes indicate that there is not an organ in that segment for a particular system.

Labeling and Drawing

Directions: You may work with your lab partner in this section.

Label the structures indicated in the drawingsbelow.


Analysis Questions

Directions: This section is to be done independently. No checking with other people – your work only.

  1. Summarize the characteristics associated with Phylum Annelida by completing the following Table. Indicate yes or no for each characteristic. (A system is several organs or parts associated with a common function.)

Characteristic of Annelids
Body Segments
Hermaphrodites
(both sets of reproductive organs present & functional)
Separate Sexes
Digestive System Present
Respiratory System Present
(lungs or gills for gas exchange)
Appendages (arms or legs)
Excretory System Present
Reproductive System Present
  1. Give the function for each of the following systems. (Consult your readings or notes if necessary.)
  2. Circulatory______
  3. Digestive______
  4. Excretory______
  5. Nervous______
  6. Reproductive______
  7. Briefly explain how an earthworm may obtain its oxygen if no respiratory system exists.

  1. Complete the following cross section outline diagram by drawing and labeling the organs that would be found in segment 10 of an earthworm. (Use the “map” as a guide.)