III.

Principles of Comparative Anatomy

Despite the variety and complexity of animal life, several key anatomical features divide up the animal world. One of these features is symmetry, meaning that an animal’s body parts are the same in size, shape, and position on either side of a dividing line or central axis. Several groups of marine animals—including the cnidarians (jellyfish, sea anemones, and corals), comb jellies, and echinoderms (sea stars, sea urchins, and their relatives)—are radially symmetrical. Their body parts are arranged around a central axis like spokes in a wheel. Almost all other animals, including vertebrates, are bilaterally symmetrical, with two halves arranged on either side of a central dividing line.

Bilateral symmetry is often not quite as perfect as it seems. The human body looks more or less symmetrical from outside, but many internal organs are arranged in an asymmetrical way. For example, the liver lies mostly on the right side of the body’s dividing line, while the stomach is mostly on the left. In some animals, asymmetry goes much further. A sperm whale has a single blowhole on the left side of its head, while a fish known as a winter flounder has both eyes on the right side. Male fiddler crabs have one small pincer, which is used for feeding, and one giant one, which is used for signaling during courtship. This giant pincer can be either on the right or the left, and it often weighs as much as the rest of the body put together.

Some bilateral animals, notably annelid worms (such as earthworms) and arthropods, show a characteristic known as segmentation. Segments, known to biologists as metameres, repeat from front to back of the animal’s body. The segments are all built on the same plan: Each one of an earthworm’s segments contains nerves, blood vessels, and excretory organs called nephridia arranged in the same pattern.

Many bilaterally symmetrical animals also show a feature known as cephalization, a trend toward 'front-end' development. Some animals with only rudimentary cephalization simply have a distinct front end that leads the way when the animal moves. But in other animals, the front region, or head, has become the part of the body that houses the brain and most of the sense organs. Particularly noticeable in arthropods and vertebrates, cephalization gives active animals the earliest possible information about food, danger, and other aspects of the environment ahead.

In comparing two species, anatomists have to be careful to differentiate between homologous structures, which are ones that have evolved from a shared ancestor, and analogous structures, which have developed from different origins. Homologous structures are built on the same underlying plan. A human arm, a bat’s wing, and a whale’s flipper look quite different from the outside, but the bones inside reveal that these limbs all have the same basic structure. Analogous structures, by contrast, often look similar, but their similarities are only skin deep. A fish's tail fin and a whale's flukes are analogous structures—they look similar from the outside and perform similar functions, but their underlying structures are quite different. Homologous structures are evidence that two species have a shared ancestry. However, analogous structures most often indicate that two unrelated species evolved in a similar environment where both developed structures to perform the same function.