Vertebrate

I

Introduction

Vertebrate, animal with a backbone, or spinal column, made of interlocking units called vertebrae. This strong but flexible structure supports the body and anchors the limbs, and it also protects the nerves of the spinal cord. Vertebrates include fish, amphibians, and reptiles, as well as birds and mammals. In all vertebrates, the spinal column forms part of a complete internal skeleton. Unlike the hard external skeleton covering an insect, which is periodically shed as the insect grows, a vertebrate’s internal skeleton can grow gradually along with the rest of the body.

Vertebrates make up only about 2 percent of the animal species, and they belong to just 1 of more than 30 phyla, or overall groups, in the animal kingdom. Despite this, vertebrates occupy a dominant position in almost all habitats and are by far the most familiar animals. When asked to name an animal at random, most people identify a type of vertebrate.

There are several reasons why vertebrates are so successful and so noticeable. One has to do with their size. Invertebrates—that is, animals without backbones, such as worms, shellfish, and insects—tend to be small and slow moving. This is because they lack effective ways to support a large body and the muscles needed to power it. Vertebrates, on the other hand, have evolved a much more versatile support system. Their skeletons can be adapted for use in many different ways and work just as well in an animal weighing 4 tons as in one weighing 113 g (4 oz). As a result, vertebrates have been able to develop bigger, faster bodies than invertebrates.

Vertebrates also have highly developed nervous systems. With the help of specialized nerve fibers, they can react very quickly to changes in their surroundings, giving them a competitive edge.

---

---

II

Characteristics

In nearly all vertebrates, bone gives the skeleton its strength. Bone is a living tissue composed of hard mineral salts produced by specialized cells. Unlike an oyster’s shell or a grasshopper’s body case, bone can strengthen after it has reached full size, and it can be repaired if it breaks. The only vertebrates that do not have this kind of skeleton are cartilaginous fish, a group that includes sharks, skates, and rays. As their name suggests, the skeletons of these species are made of cartilage, a rubbery tissue that other vertebrates have mainly in their joints.

A vertebrate's spinal column is held together by strong ligaments, but the faces of adjoining vertebrae are separated by elastic pads called intervertebral disks. These disks allow a small amount of movement at each joint, and as a result the entire spine can bend. How far the spine bends depends on the number of vertebrae that compose it and how they are shaped. Frogs, for example, can have as few as nine vertebrae, and their backbones hardly bend at all. Humans have 33 vertebrae, making us fairly flexible, and some snakes have more than 400, enabling them to shape their bodies into coils.

Besides the backbone, vertebrates share many other physical features. Their bodies are more or less bilaterally symmetrical (divisible into two equal halves), with sense organs concentrated in the head. Most vertebrates have jaws, and their brains are usually protected by a bony case called the cranium. Most also have limbs, but the shapes and uses of vertebrate limbs vary enormously. Fish typically have several paired fins and a large finned tail, but all other vertebrates either have four limbs or are descended from ancestors that had four. Four-limbed animals, known as tetrapods, use their limbs to swim, walk, run, and fly.

Although vertebrates do not have external skeletons, they often have other anatomical features that protect the surface of their bodies. Most fish and reptiles have a covering of hard scales, while birds and mammals have feathers or hair. Feathers and hair are not as tough as scales, but they have other functions apart from physical protection. One of the most important is insulation. By regulating the heat generated inside the body, such coverings allow birds and mammals to remain active in a wide range of temperatures.

Nearly all vertebrates breed by sexual reproduction, either laying eggs or giving birth to live young. The few exceptions to this rule include animals such as North American whiptail lizards, which can breed without mating in a process known as parthenogenesis. In several species of these lizards, males have never been found.

III

Types of Vertebrates

There are over 40,000 species of vertebrates, which scientists classify into five groups: (1) fish, (2) amphibians, (3) reptiles, (4) birds, and (5) mammals. Scientists divide fish into three groups based on their anatomy: jawless fish, cartilaginous fish, and bony fish. The other vertebrate groups are made up of tetrapods, which have lungs and generally live on land.

A

Jawless Fish

Jawless fish are the only living vertebrates that have never evolved jaws. There are about 50 species—a tiny fraction of the world's total fish—and they are instantly recognizable by their suckerlike mouths. Eels, lampreys, and hagfish are examples of jawless fish.

	More from Encarta
	•  7 tips for funding an online degree •  How to succeed in the fashion industry without being a top designer •  Presidential Myths Quiz •  Coffee break: Recharge your brain