Nasco's Dissection Guide for the Earthworm
DISSECTION GUIDE FOR THE EARTHWORM
The earthworm belongs to a group of animals called annelids (segmented worms). The body of an annelid is usually divided internally and externally into well-defined segments which may be separated from each other by membranous partitions. Except for the tail and head regions, all segments are essentially alike. Other members of this group include the clam worms and tube worms, which live in the ocean, and the leeches.

The earthworm hunts food at night and thus has been called a "night crawler." it usually extends its body from the surface opening of a small tunnel which it makes by "eating" its way through the soil. The rear end of the worm's body remains near the opening while the head end forages for decaying leaves and animal debris.

It has been estimated that an acre of good soil contains over 50,000 earthworms. By their continuous foraging and tunneling these worms turn over 18 to 20 tons of soil per acre and bring over one inch of rich soil to the surface every four to five years. Thus, indirectly, the earthworm enriches farmland and provides for more food in a rapidly expanding population.

A. EXTERNAL ANATOMY:
Using Figure 1 as a guide locate the following (A hand lens or dissecting microscope will be helpful in locating the smaller features.):

EXTERNAL ANATOMY

At the anterior (front) end is a small fleshy projection over the mouth. This is the prostomium. it is not considered to be a segment of the worm. The anus, the opening at the end of the digestive tract through which solid wastes are expelled, will be found at the posterior (hind) end.

About one-third of the way back from the mouth region is a thick cylindrical collar - the clitellum. This structure, involved in reproduction, will be considered later.

Place the worm so that the ventral side is uppermost. In living worms the ventral (lower) surface is a lighter color than the dorsal (upper) surface. In preserved worms the prostomium extends from the dorsal surface.

With your finger, lightly stroke the ventral surface in an anterior direction. The bristles you feel are the setae and are used by the worm in movement. How many pairs of setae are there in each segment of the worm? Does each segment have setae?

Each segment (except the first three and the last one) contains pores. The small openings connect with the metanephridia which are the primitive kidneys of the earthworm. Liquid wastes, which collect in the body cavity, are excreted through these nephridial openings.

Each segment of the worm is separated from the next one by a thin wall called a septum.

B. INTERNAL ANATOMY:
Place the earthworm on the dissecting tray, dorsal side up, and pin into position (Figure 2A). To expose the internal organs, dissect the worm as outlined in Figure 2B-F.

A. Pin through prostomium. Pin through last (anal) segment.

INTERNAL ANATOMY

B.With forceps, lift dorsal skin. Cut slit at base of forceps (a) insert scissors and cut a line, slightly off center, through to the anus (b). Caution: Be careful to cut only as deep as the skin to avoid damaging internal organs.

INTERNAL ANATOMY

C. Beginning at the anal end, hold body wall with forceps and with razor or scalpel, cut through septa on both sides of the intestine. Cut to within 1/2 to 1 inch of the clitellum.

INTERNAL ANATOMY

D. Pin the body wall to the dissecting tray as illustrated.

INTERNAL ANATOMY

E. Cut through ciitellum anteriorly. Sever the septa and pin body wall to tray as illustrated in F.

INTERNAL ANATOMY

  1. Digestive Svstem: 

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    Figure 3 shows the general location and structure of the digestive system. The other organ systems have been omitted. Locate:

    • a. The mouth is at the anterior end. The opening is located just below the overlapping prostomium.
    • b. The mouth leads to a slightly expanded and muscular pharynx which is usually covered by three pairs of whitish seminal vesicles. These will be examined later. Food taken in by the animal is passed on by muscular contractions in the pharynx through the esophagus to the crop where it is temporarily stored,
    • c. The crop opens into a thick-walled, highly muscular gizzard where food, with the aid of small soil particles taken in during feeding, is ground up. It is then passed into the intestine where digestion and absorption occur.
    • d. Solid waste products of digestion are passed to the exterior through the anus.
    INTERNAL ANATOMY
  3. Circulatory System

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    An interesting feature of the circulatory system is that it is a "closed" system in which the blood circulates within a series of blood vessels. The blood is red because it contains hemoglobin (the pigment that gives the red color to human blood).

    The major vessels of the circulatory system are a dorsal longitudinal vessel lying on top of the digestive tract and a ventral blood vessel lying below it. These two vessels are connected with each other by a number of vessels passing around the digestive tract. Five pairs of these (located in segments 7 to 11 inclusive) are larger than the others and comprise the "hearts." Pulsation of the hearts causes circulation of the blood.

    Most of the other blood vessels are difficult to observe unless the material has been sectioned for microscopic study.

  5. Reproductive System

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    To obtain a clear view of the reproductive organs, cut through the intestine near the clitellum. Carefully lift it and ease it free as far forward as the posterior end of the pharynx. Then cut it out.

    Although earthworms are monoecious (They possess a complete set of male and female reproductive organs.) they nevertheless undergo cross-fertilization during copulation. Two worms come together along their ventral sides and become temporarily joined together by the secretion of a "slime tube." During copulation sperm cells are reIeased from the seminal vesicles and stored in the seminal receptacles of the opposite earthworms The animals separate and the clitellum then secretes a mucus ring. This ring slides over the anterior segments and picks up eggs from the oviducts in the 14th segment and sperm as it slides over the anterior end of the worm to form the egg cocoon, from which the young eventually hatch. The seminal vesicles have previously been described as lying alongside of the crop and gizzard. The testes are inside of these bodies and cannot be seen. Near the seminal vesicles, in segments 9 and 10, are the four small, spherical seminal receptables. By careful examination with a hand lens or dissecting microscope you may be able to locate the single pair of ovaries in segment 13.

    INTERNAL ANATOMY

  7. Nervous System

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    The nervous system in the worm is difficult to study. Its major component is the ventral nerve cord which runs the length of the worm on the inner ventral surface (Figure 3). At its anterior end the cord divides and passes around the front part of the pharynx where it enlarges to form two swellings - the cerebral ganglia, These might be considered primitive "brains."

    Along the length of the cord, lateral nerves are given off which go to the muscles of the body wall.

  9. Excretory System

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    Each segment of the body, except the first three or four and the last one, contains a pair of excretory structures called metanephridia. These coiled tubular structures, lying next to the body walI, open to the exterior by a pore called a nephridiopore internally they are connected to the septum of the segment just anterior to them. Each nephridium collects fluid wastes from the segment anterior to the segment in which it is located.