Vertebrate Characteristics

Vertebrates are chordates that have a backbone. Classes of vertebrates include fishes, amphibians, reptiles, birds, and mammals. All vertebrate classes except fishes spend part or all of their life on land. Many characteristics of terrestrial vertebrates are adaptations to life on land and fall into two broad categories: support of the body and conservation of water.

figure 32-10

figure 32-10

(a) The legs of amphibians, such as this tree frog, Agalychnis saltator, are sharply bent and positioned away from the body.

(b) The legs of terrestrial mammals, such as this deer, Odocoileus virginianus, are straighter than those of amphibians, providing greater mobility and speed.

Segmentation and Support of the Body

Although it is not immediately apparent, vertebrates are segmented animals. Segmentation is evident in the ribs and the vertebrae (vuhr-tuh-bree), the repeating bony units of the backbone. As terrestrial vertebrates evolved from aquatic vertebrates, their limbs and associated muscles evolved to give the animals better support and greater mobility. For example, the legs of amphibians, the first land vertebrates to evolve, are positioned to the side of the body, as shown in Figure 32-10a. However, the legs of mammals, such as the deer shown in Figure 32-10b, are positioned directly beneath the body, allowing the animal to move faster and with a longer stride. Humans show an extreme version of this trait: we are bipedal, and our head is positioned directly over our body.

Vertebrates have an endoskeleton, an internal skeleton made of bone and cartilage, which includes the backbone. The endoskeleton grows as the animal grows.

Body Coverings

The outer covering of an animal is called the integument (in-TEG-yoo-muhnt). Although the integuments of fishes and most amphibians are adapted only to moist environments, the integuments of most terrestrial vertebrates are adapted to the dry conditions of a terrestrial environment. All animal bodies are composed of water-filled cells, and if the water content of the cells is reduced appreciably, the animal will die. Thus, the outer covering of terrestrial vertebrates, such as reptiles, birds, and mammals, is largely watertight. Integuments also serve other purposes. The moist skin of an amphibian functions as a respiratory organ for the exchange of gases. The scales of a reptile help protect it from predators. The feathers of birds and the fur of mammals efficiently insulate the body.

Respiratory and Circulatory Systems

Gas exchange occurs in the gills of aquatic vertebrates, including fishes and larval amphibians, but these gills do not function out of water. Lungs are organs for gas exchange composed of moist, membranous surfaces deep inside the animal's body. Lungs evolved in terrestrial vertebrates.

Vertebrates have a closed circulatory system with a multicham-bered heart. In some vertebrates, the multichambered heart separates oxygenated and deoxygenated blood, improving the efficiency of the circulatory system over that found in other vertebrates and many invertebrates.

Digestive and Excretory Systems

Digestion occurs in the gut, which runs from the mouth, at the anterior end, to the anus, at the posterior end. In many vertebrates, the gut is very long and folded, which helps increase the surface area over which nutrients can be absorbed. The human digestive tract is about 7 m (23 ft) long.

Both vertebrates and invertebrates must deal with the very toxic ammonia their bodies produce. Most vertebrates must expel wastes while conserving water. Like invertebrates, most vertebrates convert ammonia to less toxic substances. In most vertebrates, organs called kidneys filter wastes from the blood while regulating water levels in the body.

Nervous System

Vertebrates have highly organized brains, and the control of specific functions occurs in specific centers in the brain. The structure and function of the nervous system vary among vertebrate classes. For example, much of a fish's brain processes sensory information. Fishes have limited neural circuitry devoted to decision making. A fish's responses to stimuli in its environment are rigid, that is, they vary little from situation to situation and from fish to fish.

Other animals, such as dogs, display complex and flexible behavior. Much of the tissue in the dog's brain is given over to decision making, and its brain is large with respect to body size.

Reproduction and Development

In most fish and amphibian species, eggs and sperm are released directly into the water, where fertilization takes place. In reptiles, birds, and mammals, the egg and sperm unite within the body of the female, increasing the likelihood that an egg will be fertilized.

The fertilized eggs of many fishes, amphibians, reptiles, and birds develop outside the body. A developing embryo is nourished by the egg yolk and protected by jellylike layers or a shell. The zygotes of some fishes, amphibians, and reptiles remain inside the body of the female, nourished by the yolk until they hatch. In contrast, most mammals give birth to live offspring. Embryos of pla-cental mammals develop in the female's body, nourished by the mother's blood supply until the young are born. With the exception of amphibians and some fishes, vertebrates undergo direct development. So, the young and the adults can share the same resources—an advantage if those resources are plentiful.

Pregnancy And Childbirth

Pregnancy And Childbirth

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