Earthworms
Not so simple as you might think
Oren Pazgal
4/9/14
Vu-PAP Bio-1
Objectives
- Students will be learning about the external and internal anatomy of an earthworm.
- Students will focus on the organs, structures, and functions of the digestive system.
- Students will also understand the ecological role of the earthworm.
Lumbricus terrestris
Earthworms are classified under Phylum Annelida, which means that they are invertebrates, segmented, and soft-bodied. Earthworms are also some of the simplest coelomates, meaning they have a body cavity. The worm is essentially a "tube within a tube." As simple as they might seem, you might not expect earthworms to live for a full six and a half years. They reach sexual maturity at about a year of age, at which point a clitellum develops. Worms are hermaphrodites, so when it comes time to mate two worms line up their male gonads (testes) with the other's female gonads (ovaries) and release their sperm and eggs into the other worm through a mucus sheath created by the clitellum. During fertilization, the eggs and sperm will be encased in a cocoon where fertilization will occur. A young worm will break free of this cocoon after several weeks. Then it will grow, reach sexual maturity, and continue the cycle
Predators
Earthworms largest predator is undoubtedly humans, who often kill thousands and thousands of earthworms accidentally through plowing fields where the earthworms live, as well as using chemical fertilizers. Next to humans, earthworms' greatest predators are snakes, birds, rodents, and many insects, all of whom use earthworms as a food source. Fish are also quite partial to worms, the proof of which is how commonly worms are used as bait by fishermen.
Habitat
Earthworms live in cool, damp places underground where the soil has enough moisture to sustain them. Lumbricus terrestris is originally from Europe, but humans inadvertently spread them all across the globe. Earthworms are vital to their habitat because of the important work they do in enriching the soil; the hole that worms burrow function to aerate the soil and allow water to enter. However, earthworms can also be an "invasive species" in that they take away nutrients that would otherwise go to local herbaceous plants.
Eating Habits
Lumbricus terrestris eat microorganisms, detritus, and fungi contained in the soil. Earthworms essentially eat raw soil, and then filter out all of the nutrients they require before expelling the now more nutrient-rich soil back where it came from. The way they accomplish this filtration will be explored in greater detail later on.
Earthworm Evolution (Eartholution?)
One of the most important adaptations that the earthworm has evolved is the ability to respire through its skin, which allows it to comfortably live underground, which it would be unable to do if it had lungs. As an annelid, it possesses a segmented body and streamlined shape, which are both very conducive to burrowing through soil, and have also been naturally selected for their habitat. In fact, earthworms are very well adapted to underground life in all respects. Instead of eyes, ears, a nose, or ears, Lumbricus terrestris has evolved specialized sensory organs that perform the same functions without hindering its ability to function in such a confined space. Another important evolutionary milestone that the earthworm has is a coelom, or body cavity- a characteristic that places the earthworm far above its predecessors.
Ecological niche
Earthworms are detritivores, but they also function as decomposers for their ecosystems. In some cases, they serve as primary consumers as well. More importantly though, earthworms enrich and oxygenate the soil around them, which helps plant life flourish and the ecosystem to thrive.
Dissection
Labelled Guide
The picture above shows a labelled diagram of many pertinent organs and features of the anterior end of an earthworm, both internal and external. It is a reference tool for identifying the structures in the dissection pictures.
Before
Note the many segments of the worm. The tapered end is the posterior, and the more rounded end is the anterior. The large rounded bulge is the aforementioned clitellum.
***This is a dorsal view of the worm
After
A long incision has been made from the mouth to the clitellum, and horizontal incisions from the main cut allow the worm's skin to be moved out of the way. This exposes the earthworm's organs including parts of the digestive, circulatory, nervous, and reproductive systems.
Highlight: The digestive system
The earthworm consumes soil through its mouth on the anterior end of its body. The soil moves into pharynx which is the equivalent of the worm's throat. From there the soil passes through the esophagus and into the crop, where it is stored. After that, the digested soil enters the gizzard, where rocks that the worm has swallowed grind up the soil completely. The gizzard leads to the intestine, where glands release fluids that aid in digestion, until eventually the waste that had no use for the body in terms of nutrients is expelled through the anus, all the way at the posterior end of the worm.