A large percentage of animals belong to a group called vertebrates. All vertebrates are animals with backbones. They are the most complex of the three animal groups that belong to the Chordate phylum. All chordates have a notochord, which is a rod of stiffened tissue. Chordates also have a hollow nerve cord in their backs and gill slits. In most vertebrates, a backbone made of vertebra replaces the notochord as the animal develops. Vertebrates have an internal system of bones called an endoskeleton. The vertebrae, skull and other bones of the endoskeleton support and protect the animal’s internal organs; the skeleton also provides a place where muscles are attached. Vertebrates have two different ways of dealing with internal body temperature. Most vertebrates are ectotherms; whose body temperature changes with the temperature of their surroundings. Endotherms have a constant body temperature. The body temperature of an endotherm usually remains the same no matter what the temperature of its surrounding environment. 

Fish are the largest group of vertebrates living in water. Fish are ectotherms that live in water and use gills to get oxygen. Gills are fleshy filaments that are filled with tiny blood vessels. The heart of the fish pumps blood to the gills. As blood passes through the gills, it picks up oxygen from the water that is passing over the gills; carbon dioxide is released from the blood into the water. Most of the fish have fins, they are fanlike structures used for steering, balancing, and moving. Usually they are paired. Scales are another common characteristic of fish; they are hard, thin, overlapping plates that cover the skin. These protective plates are made of a bony material.

Scientists classify fish into three distinct classes. They are bony fish, jawless fish and cartilaginous fish. Bony fish have skeletons made of bone, while cartilaginous fish and jawless fish both have endoskeletons made of cartilage. Cartilage is a tough, flexible tissue similar to the bone, but is not as hard.

          About 95 % of all fish belong to the class known as bony fish. The body structure of typical bony fishes has skeletons made of bone. Their scales are covered with slimy mucus that allows the water to easily flow over the fishes’ bodies as they swim in water. The majority of bony fish use external fertilization to reproduce. The bony fish have a swim bladder, this air sac helps control the depth at which the fish swim. The exchange of gases between the swim bladder and the blood, mostly oxygen in deep water, changes the inflation of the swim bladder. As the swim bladder fills with the gases, the fish becomes more buoyant and rises in the water. When the bladder deflates, the fish becomes less buoyant and sinks lower in the water.

Few fishes belong to the class known as the jawless fish. These fish have long, scale less, tube like bodies and an endoskeleton made of cartilage. They have round mouths but no jaws. Their mouths act like suckers with sharp tooth like parts. A sea lamprey is an example of jawless fish.

Sharks, skates and rays are cartilaginous fish. These fish have skeletons made of cartilage just like jawless fish but they have movable jaws and scales. Their scales feel rough like sandpaper and they are predators.

Amphibians are vertebrates that spend part of their lives in water and part on land. They are also ectotherms, which means that their internal body temperatures change with their environment. Frogs, toads and salamanders are the most common kinds of amphibians.

Living on land is different from living in water; amphibians have behavioral adaptations that allow them to cope with swings in the air temperature of their particular environment. During cold winter months, they are inactive. They bury themselves in mud or leaves till the temperature warms up. This is called hibernation. Amphibians that live in hot, drier environments are inactive and hide in the ground where it is likely to be cooler and more humid. This is called estivation.

Amphibians have a strong endoskeleton made of bone. Another adaptation increases amphibians’ chances of survival on land. Instead of using gills to obtain oxygen from water, lungs become the primary method of obtaining oxygen from air. To increase the oxygen supply, amphibians exchange O2 and CO2 through their moist, scale less skin or the lining of their mouths. Moving to land provides an increased food supply for adult amphibians. The eardrums vibrate in response to sound and are used for hearing. Large eyes provide excellent vision. The long sticky tongue extends quickly capture the insect.

Metamorphosis is a series of body changes that occur during the life cycle of an amphibian. Most amphibians go through a two stage metamorphosis. The larval stage lives in water, the adult lives on land. The rate at which metamorphosis occurs depends on the species, the water temperature and the amount of available food. The less available food is and the cooler the water temperatures are, the longer it takes for metamorphosis to occur.

Reptiles are ectothermic vertebrates with dry scaly skin. Reptiles are vertebrates that do not depend on water for reproduction. Reptiles vary greatly in size, shape and color. Turtles are covered with a hard shell. They withdraw into the shell for protection. They eat insects, worms, fish and plants. Alligators and crocodiles are feared predators that live in and near water. Lizards and snakes are the largest group of reptiles. Lizards have movable eyelids, external ears, and legs with clawed toes. Snakes don’t have eyelids, ears or legs. Instead of hearing sounds, they feel vibrations in the ground. Snakes are also sensitive to chemicals in the air. They use their tongue to smell these chemicals.

There are two major adaptations involved when reptiles reproduce. Internal fertilization is much more efficient than external fertilization. Once fertilized, the embryo develops within the protective environment of the amniotic egg. A leathery shell provides more protection than the jelly covered frog’s egg. When hatched, the young reptiles are fully developed. Another adaptation for life on land includes a thick, dry, waterproof skin.  This skin is covered with scale and prevents dehydration and injury. All reptiles breathe with lungs.

Birds are vertebrates with two legs, two wings and bills or beaks. They lay hard shelled eggs, have feathers and are endotherms. Birds are the only animals that have feathers. The hard shelled eggs protect the developing birds. Being an endotherm, birds maintain a constant body temperature of about 400C. There are about 9000 species of birds.

Most body adaptations for birds are designed to enable them to fly. Their bodies are streamlined. If you look inside the bone of a bird, it is hollow. Flying requires that they have a rigid body. Fused vertebrae provide the needed rigidity, strength, and stability. Birds need a good supply of oxygen to fly. Efficient hearts and lungs aid in respiration. The lungs are connected to air sacs that can be found throughout the body. Air sacs make the bird lighter for flight and help bring more oxygen to the blood. The wings have large, powerful flight muscles attached to the breastbone or sternum. Birds beat their wings to attain both thrust and lift. A bird’s wing provides lift without constant beating. A bird’s wing is curved on the top; it is flat or slightly curved on the bottom. As air moves across the wings, it has a greater distance to move across the top of the wing that along the bottom. The longer path taken by the air moving over the upper surface reduces the air pressure there; as a result greater pressure is felt on the lower surface of the wing. This difference in air pressure results in lift.

A bird’s body is covered with two types of feather, contour feathers and down feather. Strong, lightweight contour feathers give birds their coloring and streamlined shape. Surface contour feathers overlap each other. This means that the bird can move more easily through the air or water.

Feather colors help identify a bird’s species and sex. They also serve as protection that helps blend some birds into their surroundings. The contour feathers on the wings and tail help the bird to steer and keep from spinning out of control. The soft, fluffy down feather provides an insulating layer next to the skin of adult birds and cover the bodies of young birds.

Feathers may be strong but they have to be kept in a good condition. Birds preen their feathers to take care of them. When they preen, they run their beaks through the feathers. Preening helps reorganize the feather and repair the breaks, or gaps in them. In addition to repairing and reorganizing feather, preening makes feather water repellent. The bird rubs its beak against an oil gland found at the base of the tail. It then rubs off the oil from its beak and onto its feathers.

Mammals share many characteristics with other vertebrates but what makes them unique? Mammals are endotherms that have hair and produce milk to nourish their young. Being endothermic is not unique, but mammals have skin covered with hair or fur. Hair is mostly made of a protein called keratin. Some mammals, such as bear, are covered with thick fur. Hair insulates the body from both cold and heat. It also protects the mammal from wind and rain. Wool, spines, quills and certain horns are made of keratin.

Mammals put in a great deal of time and energy into the care of their young. This begins at birth. Female mammals have mammary glands that form in the skin. After the birth of the young one, milk is produced and released in these glands. For the first weeks or months, the milk provides all of the nutrition that the young mammal needs.

Mammals have body systems that are able to support a wide variety of activity. Well developed lungs are made of millions of microscopic sacs called alveoli that allow the exchange of carbon dioxide and oxygen. Mammals have a more complex nervous system than other animals. The brain, spinal cord and nerves allow these animals to utilize their senses and gather info from their surroundings. They quickly sense and react to changes in the environment. All mammals reproduce by internal fertilization. Most mammals give birth to live young ones after a period of development inside an organ called the uterus.

Mammals are classified into three groups based on how their embryos develop. The duck billed platypus belongs to the smallest group of mammals called monotremes. They lay eggs with tough leathery shells.  Kangaroos, koalas, wallabies belong to the group of mammals called marsupials. They are pouched mammals that give birth to immature offspring. Their embryos develop for only a few weeks within the uterus. When the young are born, they use their sense of smell and crawl into the pouch where they complete their development. The largest numbers of mammals belong to the placental mammal group; their embryos develop in the uterus of the female. The time of development from fertilization to birth is called the gestation period. Placental mammals are named for the placenta, a sac like organ developed by that embryo that attached to the uterus. The placenta absorbs oxygen and food from the mother’s blood, the umbilical cord attached the embryo to the placenta.

Mammals have teeth that are specialized for the type of food they eat. There are four types of teeth, incisors, canines, premolars and molars. Incisors are the sharp, chisel shaped front teeth used to bite and cut off food. Herbivores have sharp incisors to grab and cut grass. Flesh eating mammals are called carnivores, they use long and pointed canine teeth to stab, grip and tear flesh. The large premolars and molars shred, crush and grind food. Some mammals eat both plants and animals. They are called omnivores.

Mammals are important in maintaining a balance in the environment. Large carnivores help control populations of grazing animals. But mammals are in trouble today. As millions of acres of wildlife habitat are developed for housing and recreational areas, many mammals are left without food, shelter and space to survive. They become endangered species.