Internal Anatomy of Earthworm

Learn about earthworms through dissection!

Earthworm Dissection Pre-AP Biology March 30, 2014- Ryan Xu

Objectives

We will dissect an earthworm and learn about the external and internal anatomy of an earthworm. We will focus on the organs and structures inside the earthworm. We will also learn about the digestive system of an earthworm.

Our Friendly Neighbor------Earthworm

An earthworm is a tube-shaped, segmented animal commonly found living in soil, that feeds on live and dead organic matter. Its scientific name is Lumbricus terrestris. They eat dead and decaying plant and animal matter from the soil, which helps aerate the soil.

Digestive system : At a glance

The digestive system is partitioned into many regions, each with a certain function. The digestive system consists of the pharynx, the esophagus, the crop, the intestine and the gizzard. Food such as soil enters the earthworm’s mouth where it is swallowed by the pharynx. Then the soil passes through the esophagus, which has calciferous glands that release calcium carbonate to rid the earthworm’s body of excess calcium. After it passes through the esophagus, the food moves into the crop where it is stored and then eventually moves into the gizzard. The gizzard uses stones that the earthworm eats to grind the food completely. The food moves into the intestines as gland cells in the intestine release fluids to aid in the digestive process. The intestinal wall contains blood vessels where the digested food is absorbed and transported to the rest of the body.

Interesting Facts

1. Earthworms come in a seemly infinite variety—around 6,000 species worldwide.

One of the most familiar of them, the sort you may see in your garden, is commonly known as the night crawler (it typically surfaces after dark), the angleworm (its makes popular bait for fishing) or the rain worm (it leaves waterlogged soil after storms).

2. Of the more than 180 earthworm species found in the U.S. and Canada, 60 are invasive species, brought over from the Old World, including the night crawler.

3. Lacking lungs or other specialized respiratory organs, earthworms breathe through their skin.

4. The skin exudes a lubricating fluid that makes moving through underground burrows easier and helps keep skin moist. One Australian species can shoot fluid as far as 12 inches through skin pores.

5. Each earthworm is both male and female, producing both eggs and sperm.

They mate on the surface of the earth, pressing their bodies together and exchanging sperm before separating. Later, the clitellum (a collar-like organ that goes around the worm’s body the way a cigar band does a cigar) produces a ring around the worm. As the worm crawls out of the ring, it fills the ring with eggs and sperm. The ring drops off, seals shut at the ends and becomes a cocoon for the developing eggs.

6. Baby worms emerge from the eggs tiny but fully formed. They grow sex organs within the first two or three months of life and reach full size in about a year. They may live up to eight years, though one to two is more likely.

Earthworm egg cases look like tiny lemons. When earthworms hatch, they look like tiny adults. Photo credit: U.S. Department of Agriculture

7. Full size for an earthworm varies among species, ranging from less than half an inch long to nearly 10 feet. The latter monsters don’t occur in U.S. backyards—you’ll have to go to the Tropics to see one of them. The homegrown versions top out at around 14 inches.

8. The glaciers that crawled across Canada into the northern tier of the lower 48 states during the most recent ice age wiped out earthworms in those areas.

In other parts of the U.S., you may find native earthworm species, but the worms living in the regions scoured by glaciers are invaders from overseas, brought here intentionally by early settlers on the assumption that the worms would improve the soil, or carried accidentally in shipments of plants or even in dirt used as ballast in ships.

9. The earthworm’s digestive system is a tube running straight from the mouth, located at the tip of the front end of the body, to the rear of the body, where digested material is passed to the outside. Species vary in what they eat, but by and large their devouring of fallen leaves and/or soil allows the worms to move nutrients such as potassium and nitrogen into the soil.

Also, worm movements within the Earth create burrows that encourage the passage of air and a loosening of the soil. Good things, right? Well, maybe not. Which brings us to 10 …

10. The northern forest evolved after the glaciers retreated, yielding an ecosystem that does not benefit from earthworms. These forests require a deep layer of slowly decomposing leaves and other organic matter called “duff” that overlays the soil. When earthworm invade these forests, they quickly eat up the duff, with the result that nutrients become less available to young, growing plants and the soil, instead of aerating and loosening, becomes more compact.