The Amphioxus Song

by Philip H. Pope

A fish-like thing appeared among the annelids one day.

It hadn't any parapods nor setae to display.

It hadn't any eyes nor jaws, nor ventral nervous cord,

But it had a lot of gill slits and it had a notochord.

Chorus:

It's a long way from Amphioxus. It's a long way to us.

It's a long way from Amphioxus to the meanest human cuss.

Well, it's goodbye to fins and gill slits, and it's welcome lungs and hair!

It's a long, long way from Amphioxus, but we all came from there.

It wasn't much to look at and it scarce knew how to swim,

And Nereis was very sure it hadn't come from him.

The mollusks wouldn't own it and the arthropods got sore,

So the poor thing had to burrow in the sand along the shore.

He burrowed in the sand before a crab could nip his tail,

And he said "Gill slits and myotomes are all to no avail.

I've grown some metapleural folds and sport an oral hood,

But all these fine new characters don't do me any good.

(chorus)

It sulked awhile down in the sand without a bit of pep,

Then he stiffened up his notochord and said, "I'll beat 'em yet!

Let 'em laugh and show their ignorance. I don't mind their jeers.

Just wait until they see me in a hundred million years.

My notochord shall turn into a chain of vertebrae

And as fins my metapleural folds will agitate the sea.

My tiny dorsal nervous cord will be a mighty brain

And the vertebrates shall dominate the animal domain.

(chorus)

Now that you have some idea of what a primitive chordate was like, let's return to my earlier comment that larval forms of echinoderms have similarities to primitive chordates. unlike adult echinoderms, which are radially symmetrical (think of a starfish, where body parts radiate around a central axis), echinoderm larval forms are bilaterally symmetrical like chordates. In terms of embryology, echinoderms and chordates have a number of developmental similarities that set them apart from other bilaterally symmetrical animals. One hypothesis for chordate origins is that the larval form of an early echinoderm may have become sexually mature without growing up— that is, without going through the full metamorphosis to an adult. This phenomenon is uncommon, but it is not unknown. It occurs in salamanders such as the axolotl, for example.

In the Middle Cambrian is a small fossil called Pikaia, which is thought to be a primitive chordate because it looks rather like amphioxus (Figure 2.3). A new marine fossil discovered in the Late Cambrian Chengjiang beds of China might even be a primitive vertebrate. Although Haikouella swam, it certainly didn't look much like a fish as we think of fish today; it more resembled a glorified amphioxus (Figure 2.4). From such primitive aquatic chordates as these eventually arose primitive jawless

Figure 2.3

Pikaia, a Middle Cambrian fossil, shows some characteristics of primitive chordates. Courtesy of Janet Dreyer.

Figure 2.3

Pikaia, a Middle Cambrian fossil, shows some characteristics of primitive chordates. Courtesy of Janet Dreyer.

Figure 2.4

Haikouella, a Late Cambrian marine fossil, may be a primitive vertebrate. Courtesy of Janet Dreyer.

fish, then sharks and modern fish, and eventually the first land vertebrates: tetrapods (which means "four footed"). These in turn became the ancestors of the other great groups of land animals, reptiles and mammals. Later, more detail will be provided about the evolution of many of these groups. But it is worthwhile to first present four basic principles of biological evolution to keep in mind as you read the rest of the book: natural selection, adaptation, adaptive radiation, and speciation.

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