Nematodes abound Tiny round worms are found everywhere on Earth: the Arctic and Antarctic, the depths of the ocean and on the tops of towering mountains. Nematodes are perhaps the most abundant animal group on the planet. © William H. Amos / Bruce Coleman, Inc.

Scientists used to group arthropods and annelids together, but investigations at the DNA level enlightened their thinking. New data can refine and change ideas about branches on the evolutionary tree.

Until the early 1990s, we thought that animals with segmented bodies were closely related. That meant that arthropods and annelids were grouped together. But genetic studies, combined with developmental data, uncovered the deeper truth, buried within the DNA of seemingly unrelated arthropods, nematodes, water bears and a few other organisms.

When a lobster grows out of its outer layer, it sheds its shell and produces a new protective coating, called a cuticle. A tiny water bear, which looks nothing like a lobster, can also regenerate its outer cuticle. So can organisms in a group called nematodes. On a hunch, researchers compared the DNA from different types of molting species—arthropods, water bears and nematodes, for example. They found key similarities. The evidence suggested that the Ecdysozoa—animals that shed their cuticles—all emerged from a common ancestor.

EVOLUTION: Nervous Discord
Scientists used to think that there was a clear evolutionary trail from animals with primitive nerve chords up to vertebrates with an advanced nervous system. The history of this trait leads back to the emergence of the nerve chord in the tunicate urochordates, which originated some 570 million years ago. But what came before the chordates is not so straightforward.

Having a primitive nerve chord and gill slits, a group called the hemichordates seemed like an obvious ancestor to the more advanced chordates. But it turns out that hemichordates are more closely related to spiky sea life called echinoderms. Hemichordate acorn worms and echinoderms, such as sea urchins, look vastly different as adults. But their larvae look similar. Genetic research in the 1990s confirmed that hemichordates are more closely related to echinoderms than to chordates.

	Borrowing defenses Sea slugs are mollusks that eat anemones and other cnidarians, which have stinging cells. Not only do the slugs not get stung, they incorporate the stinging cells into the brightly colored appendages on their backs for defense. © Norbert Wu / www.norbertwu.com

Larval Link
From giant clams to bristle worms, mollusks and annelids come in all shapes and sizes. Researchers thought these animals were not closely related—until they took a closer look.

At first glance, the animals in the annelidan and molluskan groups look like they have little in common. In fact, adults within each group hardly resemble each other. Snails, oysters and squid are just a few different types of mollusk, for example. Meanwhile, segmented tube worms and bristle worms belong to a large annelid group with species that survive in a range of temperatures and habitats.

A peek at the developing larvae of each group told researchers that the younger generation has much more in common. Larvae from each of these groups have a tuft as well as two rows of cilia. Called trochophores, these larvae link together mollusks and annelids—and other groups that include spoonworms, ribbonworms and peanutworms—within the larger group called Trochozoa.