Animal Feeding and Breathing: The word animal refers to one of the two great kingdoms (groups) of living things; the other is the plant kingdom. Usually, it is easy to say by just looking whether or not something is an animal or not. But there are many kinds of living things that had to be investigated scientifically before it was possible to say quite definitely that they were animals.
An important difference between plants and animals lies in the way they feed. Plants build up foodstuffs from simple materials in the soil. But animals need more complicated foods that have been produced by plants and are contained within them. Animals take these in either by eating the plants themselves or by eating other animals that do, for instance, lions prey on antelope that live on grass. Other differences are that unlike plants most animals can move about, and animals have a nervous system that sends messages to and from the brain. Plants have no such system.
How Animals Feed
Animals eat in many different ways. The starfish pushes its stomach outside itself and wraps it round prey such as a mussel. Tapeworms, which live as parasites inside other animals, have no mouth; instead, they absorb food from the other animals through their bodies. Many water creatures feed on very tiny specks of food which they filter from the water in various clever ways. For instance, some caddis-fly larvae that live in streams build a tube to live in. They spin a small web across the top of the tube and when hungry make a stream of water go through the tube by waggling the body. Smaller creatures and algae are caught and eaten, and when the caddis-fly has had enough, it eats the web until the time comes to spin another.
Many of the smaller animals, and especially insects, live by sucking blood from other animals. To do this, they have mouths like hollow tubes with sharp points. Flies, mosquitoes, and ticks feed in this way. Similarly, many insects, such as the big cicadas or the much smaller greenflies, suck the sap from living plants. Spiders and scorpions also take only liquid food; their method is to kill their prey first, push in digestive juices and then suck out all the nourishment when it has liquified.
The vertebrates (animals with backbones) have a more complicated way of digesting their food. Snakes eat only living animals and their eggs, and always swallow them whole. Very powerful juices in their stomachs can digest everything except horns, hoofs, and hairs. Even bones disappear completely. Birds eat food in pieces, and, since they have no teeth to chew with, most have a special part of the stomach called the gizzard, where the stones and grit they swallow are used for grinding up food.
Food eaten by mammals including humans follows a long and complicated route through the digestive system. First, it is chewed up and mixed with saliva in the mouth. Then it passes to the stomach, where more juices are added and where it is well mixed for several hours until it becomes liquid. Next, it passes on, a little at a time, to the small intestine, at which stage the liver and a gland called the pancreas add more digestive juices. As the food flows on, part of it passes through the outer wall of the intestine into the bloodstream. The rest flows on down the large intestine, which is wider and shorter than the previous section. Here much of the water is absorbed and the remainder, made up of fibers and another indigestible waste matter is got rid of through the anus in the form of solid material.
Higher animals need three main kinds of food: fats, such as butter; carbohydrates (sugar and starch); and proteins (milk, lean meat, and fish). They also need certain minerals, such as calcium to develop and maintain the bones, and phosphorus; And they must have vitamins, too. For these complicated chemicals are essential for good health. All of them are found in living plants except for vitamin D, which is made in the animals’ bodies by the action of sunlight.
Breathing and waste products:
Animals get warmth and strength by using up foodstuffs. Actually, they burn them inside their bodies and for this, they need oxygen from the air. Insects generally take air in and out through openings in their bodies. Fish have gills that absorb the oxygen dissolved in water. Amphibians (when adult), reptiles, birds, and mammals have lungs and get oxygen by true breathing.
All food consists mainly of carbon and hydrogen; and when it has been burnt a lot of carbon dioxide and water are left which have to be got rid of. They both escape as gas by the same route as the oxygen came in. That is why an unventilated room with a lot of people in it gets stuffy and the windows steamy.
When proteins have been burnt, they leave behind the complicated chemicals containing nitrogen. These are got rid of by the kidneys, which are really filters. About 200 liters (50 gallons) of blood passes through the kidneys every day and filters out the substances that are not needed. These go down to the bladder and then pass with the unwanted water as urine. Birds and reptiles get rid of them in a more or less solid form, however.