Download Chapter Seven: Phylum Annelida

Chapter Seven: Phylum Annelida
The segmented worms
Who are the annelids?
For once, we are going to discuss worms and they are not all parasitic. Of course, leeches are
annelids but they are the parasitic exception. Annelids are segmented worms that often have little
bristles called setae protruding out. There are about 15,000 species of annelids and they can be found in
freshwater, saltwater or moist soil. They need a moist environment because gases diffuse in and out of
their skin directly and this can only happen when it is moist. Worms aren’t real happy in dry spots. They
are bilaterally symmetrical and have a true coelom. However, they take the coelom one step further
than the mollusks. Annelid coeloms are divided into separate compartments. How does this help them?
Well they can contract part of their body without contracting the whole thing. This helps them move in
their habitat (think inchworm).
The annelids are divided into three classes based on the amount of setae or bristles they have.
Oligochaetes have few bristles (oligo means few so this makes sense). These worms mainly live in fresh
water or the soil. The most familiar oligochaete you know is probably the earthworm. Polychaetes have,
you guessed it, many bristles on the outside of their body. Poly means many so this also makes sense.
They mainly live in salt water and some can be ferocious predators. The last class is the hirudinea.
These have no bristles and are the leeches. Not all leeches are parasites by the way. Some are
carnivorous. (What’s the difference between parasitic and carnivorous? Carniverous leeches kill and eat
their prey, parasitic leeches attach to their prey and suck its blood but usually don’t kill it.)
Christmas Tree Worm (polychaete)
Earthworm (oligochaete)
Leech (hirudean)
More on Earthworms:
Since the earthworm is the most well-known annelid, let’s take a closer look at how they are put
together.
Aortic arches
Earthworm body structures and functions:
Let’s follow some food through an earthworm. The earthworm takes in its food through its mouth
of course. It gets sucked into the mouth by the pharynx. By the way, what does an earthworm eat?
They suck in soil and feed on the organic material found in the soil (like dead leaves). The soil then gets
moved to the crop, kind of storage spot for undigested food. After enough food builds up in the crop, it is
pushed through a tube called the esophagus and forced into the gizzard which grinds it up and releases
the nutrients for the worm to use. It keeps getting pushed through the intestine which absorbs digested
material until any undigested material left over is pushed out of the anus. This so called “worm poop” is
great for the soil. It is rich in nutrients and helps aerate the soil. The food is pushed through with the help
of the setae, those bristles on the outside of the worm. The setae anchor part of the worm to the ground
and give the body muscles something to push against. As one part of the worm gets longer, it pulls the
part behind it closer. This not only helps the worm move but also helps move the food inside of the
worm.
How does the worm move gases and fluids throughout its body? Gases can move in and out of the
earthworm directly through the skin where it then enters a closed circulatory system. This moves the
blood from one end of the worm and back again. The worm has 5 “pseudo hearts” or aortic arches to
help pump blood throughout the worm. The blood moves first through ventral (bottom) blood vessel and
then returns to the “hearts” through the dorsal (top) blood vessel. Obviously the earthworm needs some
way to get rid of wastes including excess water, so how do they do it? Most segments on the earthworm
have nephridia which are a kind of primitive kidney filtration system. The nephridia reabsorb material
that is still useful and release the waste through pores on the worm’s skin.
Can earthworms think? Not like us certainly but they are very capable of reaction to external
stimuli like light, touch or moisture. They use their cerebral ganglia to process all of the sensory
information they get about their environment and respond accordingly. They are very sensitive to light
and will immediately try to burrow back underground if you dig them up.
Photo Credits:
Earthworm anatomy and dissection photo: http://io.uwinnipeg.ca/~simmons/16labman05/lb6pg2.htm
Christmas tree worm: http://www.flmnh.ufl.edu/fish/Gallery/Descript/ButterflyReef/ButterflyReef.htm
Earthworm: http://keep3.sjfc.edu/students/lmm0892/e-port/unitplan/history.htm
Leech: http://news.nationalgeographic.com/kids/2003/10/maggotsandleeches.html