HOW ARE EARTHWORMS CONSTRUCTED?

Here are some visible and invisible adaptations to admire about earthworm physique:

  • The worm's streamlined body with no conspicuous appendages is an adaptation to living in narrow burrows underground. In those narrow tunnels, antenna and fancy tails would just get in the way.
  • Earthworms don't have eyes, but they do have light-sensitive cells scattered in their outer skin. These cells don't enable earthworms to see images, or forms, but they do give their skin the capacity to detect light and changes in light intensity. The worm's skin cells are also sensitive to touch and chemicals
  • Earthworms have simple brains which specialize in directing body movement in response to light, and not much else. To show how simple the brain is, if an earthworm's brain is removed, changes in its general behavior are hardly noticeable
  • Earthworms, as the drawing above shows, possess five hearts. The calciferous glands shown between the hearts manage excess calcium in the worm's diet, a problem you may have if you eat a lot of dirt.
  • Earthworms have no lungs. Their "breathing" consists of oxygen from the air passively diffusing through the skin into the body. One problem with having such permeable skin is that earthworms dry out easily, but not as easily as mollusks
  • Each earthworm carries both male and female reproductive parts --they are, like snails and slugs, hermaphroditic

WHEN CONDITIONS GET ROUGH....

When the soil gets very cold, very hot, very wet or very dry, it's hard on an earthworm. Survival strategies vary from species to species. A typical behavior is to tunnel deeper into the soil, to where conditions aren't so extreme. Some species, especially during hot, dry periods, do what's shown at the right. At a fair depth they roll themselves into balls and excrete mucous around themselves, creating a kind of cocoon. In such a state their body functions slow down drastically. They go into a kind of suspended animation, just waiting for soil conditions to improve. When an animal goes into such a state in order to survive summer's hot, dry conditions, it's said that they estivate or "go into estivation." I found the earthworm shown above one day as I worked in the very dry soil of a garden. The earthworm was inside a very hard, dry clod of soil. While the picture was being taken it began moving, so this worm was very much alive, just "resting" until a good rain.

HOW DO EARTHWORMS TRAVEL?

The secret to earthworm travel lies in two things you can't see just by looking.

  • Though earthworms have no bones, their complex system of muscles enables them to not only wiggle like crazy but also to very quickly alternate between being stubby and thick, and long and slender
  • Earthworms possess tiny, practically invisible bristles, called setae (pronounced SEE-tee; singular form seta, pronounced SEE-tah), which usually are held inside their bodies. When the worms want to stay in their burrows, they jab their setae into the surrounding dirt, thus anchoring themselves in place. This comes in handy if a bird nabs a worm's head and tries to pull the worm from its burrow. The setae anchor the worm so well that it may break before coming out.

Here's how these two features enable earthworms to travel:

Let's say that a worm in its burrow wants to move forward. First, using its complex musculature, it makes itself long. Then it anchors the front of its body by sticking its front setae into the soil. Now it pulls its rear end forward, making itself short and thick. Once the rear end is in place, the front setae are withdrawn from the soil, but setae on the rear end are stuck out, anchoring the rear end. Now the front end is free to shoot forward in the burrow as the worm makes itself long and slender. Then the whole process is repeated.

HOW DO EARTHWORMS REPRODUCE?

This matter is as complex as it is interesting, so put your mind in low gear and hold on... Here's a drawing to help you keep oriented:

First of all, not every earthworm segment bears sex organs. Counting from the front, the worm's male sex cells lie in segments 10 and 11. From here the sperm pass through sperm ducts to two male genital openings on the bottom of segment 15. On segments 9 and 10 there are two minuscule sacs called sperm receptacles, or pores, where, during earthworm sex, sperm are deposited. However, this is not where eggs are produced. The egg-producing ovaries reside in segment 13, from which eggs are released through the female pores into egg sacs in segment 14. Finally, there's a rubbery, arm-band-like thing covering the worm's body from segments 31 or 32 to 37, and this is called the clitellum.

Now, when two earthworms mate, they line up next to one another with their "heads" pointing in opposite directions. The clitellum of one worm lies opposite segments 9-11 of the other, which, you'll remember, contain the male parts. The worms now secrete tremendous amounts of mucus, until each is enclosed in a slime tube extending from segment 9 to the rear end of the clitellum at segment 37. Now sperm are ejaculated from segment 15 and carried backward in tubes formed by grooves in the body touching the slime tube and the sperm pass to the sperm receptacles on segments 9 and 10 of the other worm.

Then the earthworms go their different ways. Sex is done. However, that is not to say that eggs have been fertilized, because they haven't. It's just that now, in each worm's sperm receptacles, there are sperm from the other worm.

The next step in producing baby earthworms comes when the earthworm is by itself. Its clitellum secretes a second mucous ring that slides forward over the worm's body. When the ring passes the openings in segment 14, several ripe eggs leave the body and stick to the ring. The ring keeps moving forward until it passes the sperm receptacles in segments 9 and 10, and here sperm come into contact with eggs. Finally, within the mucous ring, the eggs are fertilized.

Now the ring containing the fertilized eggs slips off the worm's "head," seals at both ends, and becomes a sort of cocoon, which is left lying in the soil. Ultimately the eggs hatch and tiny worms escape from the mucous ring, into the soil.

Result Questions for Earthworm Dissection

1.) What are the major organ systems that you were able to find in your earthworm?

2.) What organ system does the heart belong to, and what is its purpose?

3.) What organ system does the kidney belong to, and what is its function?

4.) Name the organs of the digestive system. How do earthworms extract the nutrients from their food?

5.) What organ system is most closely connected to the respiratory system? Why?

6.) Draw and label the basic nervous system of an earthworm. What is its major function?

7.) Why is it important for the skin of the earthworm to remain moist at all times?

8.) Why do earthworms come out of their burrows after a heavy rain?

9.) Describe the location and position of the setae. How do they aid in locomotion?

10.) Why is an earthworm’s blood red like a human’s?

11.) Why is hermaphroditism an advantage for this species?

12.) Why is the earthworm an important component of the ecosystem?