Segmented Worms

In 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 snails:

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.

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Conrad, Jim. Last updated . Page title: . Retrieved from The Backyard Nature Website at .