Earthworm Anatomy

Background — Phylum Annelida:

(annel = a little ring, a ring)

Annelid classes include

• Class Oligochaeta (oligo = few, scant; chaeta, setum = bristle), earthworms with few setae;
• Class Polychaeta (poly = many), marine worms with projections from each segment (parapods — para = beside, near; poda = foot) which serve as gills and bear many setae;
• Class Hirudinea, leeches.

Annelids are segmented, both inside and out. They have a tube-within-a-tube body plan, a closed circulatory system with five pairs of “hearts,” and a ventral, solid, nervous system which arises from mesoderm tissue. (Compare this with the vertebrate nervous system which is dorsal, hollow, and of ectodermal origin.). Many of the internal organs, for example, the paired nephridia (nephri = kidney) which serve an excretory function analogous to our kidneys, are repeated in each segment. Annelids have both longitudinal and circular muscles, so have better control over their movement than the Nematodes (which only have longitudinal muscles) do.

Leeches are “famous” for their diet: they suck blood. Some have sharp jaws to slit their host’s skin, while others secrete an enzyme to digest a hole in the victim’s skin. It is to the leech’s advantage if the would-be victim remains unaware of the leech’s presence, thus most leeches secrete some kind of anesthetic (an = not, without; aesthet = sensitive, perception) so the host does not feel their attack. To prevent the blood from clotting before they ingest it, leeches also secrete an anticoagulant (anti = against, opposite; co- = with, together; ageve = to move, put in motion; coagulum = rennet). For years, people thought disease was caused by too much blood, so the Medicinal Leech Hirudo medicinalis was used to suck some out. Typically a medicinal leech, once attached, may suck for a couple hours, perhaps ingesting between one to two ounces of blood, many times its own body weight. While we now have different theories as to the cause of diseases, medicinal leeches still have uses in modern medicine! When someone severs a finger which must be surgically reattached, since all the capillaries are dysfunctional, blood flowing into the finger via the reconstructed arteries has nowhere to go but into the tissue. This leads to problems with edema in the area. Also, because of this poor circulation, the reattached tissues cannot get the air and nutrients they need to heal properly. It has been discovered that letting a leech suck on the end of the reattached finger will a) help reduce edema, and b) create a sort-of blood flow that will allow nutrients and air to get to the reattached tissues so they heal better and more quickly. Additionally, the anticoagulant secreted by the Medicinal Leech is very powerful, and a leech bite (in a person whose blood usually clots normally) can take a couple days to stop bleeding and form a scab. Research is being done on using this powerful anticoagulant to help heart attack and stroke victims whose problems are caused by blood clots.

Marine worms have a pair of gill-like structures called parapods projecting from the sides of each segment. It is believed that the early annelid ancestors of insects (and other arthropods) may have looked like this (and the parapods later became modified into true legs).

Background — Earthworms:

In this lab, you will become familiar with the external and internal anatomy of the earthworm, Lumbricus terrestris.

As members of Phylum Annelida, earthworms have bilateral (bi = two, later = the side) symmetry and a “tube-within-a-tube” body plan. A distinguishing feature of this phylum is the division of the body into segments. This segmentation is both external and internal with many structures/organs repeated in each segment. The body has three tissue layers:

• ectoderm (ecto = outside, out, outer; derm = skin)
• mesoderm (meso = middle)
• endoderm (endo = within, inner)

The coelom (coel = hollow) is a cavity between the layers of the mesoderm. The circulatory system is a closed circulatory system, meaning that the blood remains within the blood vessels (in an open system, the “blood” bathes the body organs for at least part of its journey).

Earthworms live in the soil, working their way through it to ingest and digest organic matter within the soil. They play an important part in aerating and fertilizing the soil.

Earthworms exchange air (“breathe”) through their moist skin, thus if a worm dries out, besides the dangers of dehydration, it can’t get air so it dies. However, earthworms also cannot live underwater, and if the soil in which they are living becomes totally filled with water during a heavy rain, they will all come to the surface so they don’t drown (and end up on the sidewalk where they subsequently dry out after the rain stops).

Earthworms are hermaphroditic and a pair of worms fertilize each other.

External Anatomy:

Obtain a dissecting tray (Please note that this is not a toy. Try to avoid unnecessary pin holes in the wax and DO NOT CARVE, chop, or otherwise mutilate the wax.) and a set of dissecting tools, as well as your dissecting scope and its light. (Eventually, you will also need your “regular” microscope.) Also, obtain a rinsed, preserved earthworm. As you do your dissection, compare what you’re seeing in your worm with the large worm model. Draw a labeled illustration of and take notes on everything you view.

External View, Four Pairs of Setae
Labeled Setae

1. Feel the bristly setae (seta = bristle) on each segment, along the sides and bottom. Note that there are four pairs. Two pairs are on the sides (laterally — later = the side) and two pairs are on the bottom (ventrally — venter = the underside, belly).
2. Note the conspicuous swelling near the anterior (the front — ante = before) end called the clitellum (clitell = a pack saddle). The smooth side (often darker) is the top (dorsal — dorso = the back) and the segmented side (often lighter) is the bottom (ventral).

External View, Mouth
Labeled Mouth

3. On the very anterior tip of the worm is a projection called the prostomium (pro = before, in front of; stoma = mouth). The mouth lies ventrally between this and the first segment. Count the number of segments between the front of the worm and the front edge of the clitellum, as well as the number of segments included in the clitellum.

External View, Anus
Labeled Anus

4. On the posterior (the rear — post = behind, after) end, note the slit-like anus in the last segment.

External View, Genital Openings
Labeled External Genitalia

5. On the ventral (bottom) surface of segment 15, note the pair of (larger) openings of the vasa deferentia. On the ventral surface of segment 14, you can see the (smaller) openings of the oviducts (vasa = a vessel, duct; deferens = to carry away; ovi = egg).

Internal Anatomy:

1. On the dorsal surface of the earthworm, beginning at the clitellum, cut a slit posteriorly (toward the rear) for about 25 segments. You must cut very shallowly to avoid cutting the internal organs. Turn the scissors and cut anteriorly to the prostomium, again, being careful not to cut the internal organs.

Internal View, Dorsal Blood Vessel
Labeled Dorsal Blood Vessel

2. Pin the worm to the tray near the anterior and posterior ends and every five segments (5, 10, 15, etc.) along the body. You may have to carefully remove/cut the septa (septum = a fence) separating the segments to open up the body wall. The dorsal blood vessel and digestive tract should be exposed at this point.

Internal View, Crop and Gizzard
Labeled Crop and Gizzard

3. Examine the digestive tract. From the mouth back, locate the pharynx, esophagus (eso = within, inward; phago = to eat), crop, gizzard, and intestine. Refer to these photos, and the large earthworm model.

Internal View, Five Pairs of Hearts
Labeled Hearts

4. Examine the circulatory system. Locate the dorsal blood vessel, smaller segmental vessels coming from it, 5 pairs of hearts in segments 7-10 (count from the placement of the pins), ventral blood vessel (you may have to carefully snip a piece of intestine and hold it up to see this).

Internal View, Nephridia
Labeled Nephridia

5. To the sides/under the digestive tract are the paired nephridia (nephr = kidney), which may be too small to see well with the unaided eye.

Internal View, Seminal Vesicles and Receptacles
Labeled Seminal Vesicles/Receptacles

6. The reproductive systems are located under the digestive tract in approximately segments 10-15. Earthworms are hermaphroditic (Hermes = messenger god — Mercury; Aphrodite = goddess of love — Venus), that is, they have both sexes and when they mate, they fertilize each other. Seminal vesicles (male organs) are large, floppy, whitish structures in segments 9-13. The tubes from them to segment 15 are the vasa deferentia. Attached to the anterior septum of segment 13 is a pair of whitish, grape-clustered ovaries (female organs) which are very difficult to see. The seminal receptacles, sperm-storage areas within the female reproductive tract, are smaller, whitish organs near the seminal vesicles. Oviducts start at the ovaries, go past the seminal receptacles, then to segment 14, from which they open to the outside.

External View, Ventral Nerve Cord and Ventral Blood Vessel
Labeled Nerve Cord & Blood Vessel

7. Note the ventral nerve cord with segmental ganglia (ganglion = a knot on a string). If you lift a section of the digestive tract/blood vessels, the nerve cord should be seen lying on the ventral surface of the body cavity.

Internal View, Ventral Nerve Cord and Blood Vessel
Labeled Nerve Cord & Blood Vessel

Internal View, Brain
Labeled Brain

8. If you look carefully at the nerve cord, you may be able to see that it is actually two, paired cords. As you proceed anteriorly, the “last” segmental ganglion (the subpharyngeal ganglion) will be found in segment #4. From there, the two “halves” of the nerve cord (the circumpharyngeal connectives) split and go around either side of the pharynx. Above the pharynx, they come together, again, to form the suprapharyngeal ganglion, also called the brain. To aid you in finding your worm’s brain, Dr. Fankhauser has described the worm brain as looking like the top half of a woman’s bikini bathing suit.

Internal View, Brain
Labeled Brain

Cross-Section of Earthworm Segment

Cut a thin cross-section through your worm’s intestinal area and view it with a dissecting scope. Also, view the prepared slide of an earthworm cross section (Carolina #Z1250), and examine the large model worm.

Cross Section, Whole Worm
Labeled Worm Cross Section

1. Note/draw the layers in the body wall from the outside in: cuticle secreted by the epidermis, epidermis (epi = upon, over, beside) — made of ectoderm tissue, then circular muscle layer, longitudinal muscle layer, and parietal peritoneum (pariet = a wall; peri = around; ton = something stretched; -eum = a place where; peritoneum = the membrane around the intestines) — all mesoderm tissues (meso = middle). Note the four areas where the setae are located, although you may not see the actual setae on your slide.

Intestinal Area, x.s., Actual View
Intestinal Area, x.s., Microscopic View

2. Note the coelom (coelo = hollow), the body cavity. Often there is a tangled network visible within the coelom which is part of the nephridia, the excretory organs. Each nephridium opens to the outside via a nephridiopore (which may not be visible on your slide).

Cross Section, Central Tube
Labeled Intestinal Cross Section

3. Examine the central tube. Note that its upper (dorsal) surface is folded into the digestive tract. This portion is called the typhlosole (typhlo = blind; solen = channel, pipe), and helps to increase the surface area of the digestive tract. (Note: if you carefully open a length of your worm’s intestine (and clean out the soil/food contained therein), you may be able to see the typhlosole running the length of the dorsal surface and somewhat resembling a tightly-coiled spring.) The layers of the central tube, from outside in, are the chloragen cells, then thin muscular layers (both mesoderm), then the gastric epithelium (gastro = stomach, theli = nipple), which is endoderm tissue, as the lining of the digestive tract. The space within the digestive tract is known as the lumen. Dorsally and ventrally of the digestive tract run the dorsal and ventral blood vessels. Ventrally, in the coelom, there is a nerve cord that runs the length of the body.

Other Things to Include in Your Notebook

Make sure you have all of the following in your lab notebook:

• all handout pages (in separate protocol book)
• all notes you take as you read through the Web page and/or during the introductory mini-lecture
• all notes and data you gather as you perform the lab
• labeled drawings (yours!) of
  • overall external anatomy
  • external view of anterior end
  • external view of posterior end
  • external, ventral view of segments 14-15
  • external arrangement of setae
  • overall internal anatomy (dorsal view)
  • “close-up” of any internal organs of special interest, such as the “brain”
  • cross-section of intestinal segment
• answers to all discussion questions, a summary/conclusion in your own words, and any suggestions you may have
• any returned, graded pop quiz