n our backyards the main segmented worms, or annelids
are earthworms. If you've ever looked closely at earthworms, you've probably decided that
they're pretty simple affairs, maybe even less sophisticated than snails and slugs.
However, that's not necessarily the case. Their anatomy indicates that, in evolutionary
terms, annelids are more advanced than mollusks
. Here are some of the ways annelids are more complex -- more "highly evolved" -- than mollusks such as slugs and
HOW ANNELIDS ARE
MORE COMPLEX THAN MOLLUSKS
- Annelids are segmented, with each segment bearing the same fundamental structures
as all the others, though minor differences can occur between some segments. By
distributing organs among many segments, it becomes less dangerous to an annelid if one
organ is damaged.
- Young annelids usually add new segments as they grow older by simply making new
copies of the body's last segment -- a sort of efficient assembly-line construction
- In annelids, blood circulates in a closed system of blood vessels -- it does not
at some point simply drain into open sinuses, as with the mollusks. This assures that the
annelid's blood doesn't pool someplace in its body and for a time become useless, and that
only oxygen-depleted blood is circulated back to have its oxygen replenished
- Annelids are covered with a very thin, cellophane-like cuticle, which cuts down
on moisture loss from the body. Annelids don't dry out as fast as mollusks
Despite these "improvements" over mollusks, annelids are by no
means very sophisticated. They don't have anything as fancy as a skeleton, or an ability
to keep their bodies warm when the temperature drops. Also, the problem with their bodies
drying out remains critical, despite the cuticle. They don't have very big brains, either.
Here's something very interesting about segmented worms: If an explorer- naturalist
from another galaxy came to Earth with the assignment of collecting an "average
Earth-animal," after studying everything from the lowliest sponges, jellyfish, and
flatworms, to us humans, there would be a fair chance that he/she/it would choose to take
back an annelid. In other words, segmented worms stand about midway between the simplest
and most complex of Earth's animals.
For an introduction to the annelids, you may be interested in reviewing at Amazon.com
the book Earthworms,
Leeches, and Sea Worms: Annelids.