An Introduction to Zoology: Chapter 11

Phylum Acoelomorpha:

Members of the phyla Acoelomorpha and Platyhelminthes are commonly known as “flatworms.” The “flat” part is self-explanatory, but what is a “worm”?

Well, the acoelomorphs are the first animal phylum we’ve discussed whose members have elongated, bilaterally-symmetrical bodies. They’re also the first animals we’ve discussed that show any degree of cephalization. These are two important characteristics of “worms.”

A typical cnidarian or ctenophoran isn’t especially active. For such an animal, radial symmetry seems to work just fine, since a radially-symmetrical animal can sense danger (or prey) from any direction. The disadvantage is that radially-symmetrical animals are almost invariably slow-moving or completely immobile.

An animal that actively seeks food, shelter, mates, and so forth requires a body that is constructed differently. Active movement is most efficient if the body is elongated, with distinct anterior (front) and posterior (rear) ends. As such, selection for more efficient movement was probably what led to the evolution of bilateral symmetry. An elongated body is most efficient if one surface of the body is specialized for movement. So, the great majority of bilaterally-symmetrical animals have a ventral (belly) surface that is specialized for locomotion, and a dorsal (back) surface where most of the sense organs are located.

Naturally, it’s better to see where you’re going than where you’ve been, so once bilateral symmetry evolved, there would be selection for concentrating the sense organs and neural tissues at the animal’s anterior end. Concentration of neural tissues at the animal’s anterior end allows the formation of a brain of some sort, that can integrate sensory information and coordinate body movements. Having your neural tissues concentrated near the sense organs also allows for quicker responses.

This concentration of neural tissues and sense organs at the anterior end of the animal’s body is cephalization. Virtually all the animals we’ll discuss from now on are bilaterally symmetrical and show at least some degree of cephalization.



“But what is a worm?” I hear you ask. Well, a “worm” is simply any invertebrate animal that has an elongated, limbless body. In modern usage, a “worm” is always an invertebrate, but originally the word referred to any animal with an elongated and more or less legless body. So, if you read old tales, you’ll sometimes see dragons called “worms” (or “wyrms”). Similarly, if you visit the southern Appalachian Mountains, you can still run across people who call rattlesnakes “buzzworms.”