The study of the structure of living things is called anatomy. All animals are made up of CELLS, some of which are specialized to carry out different functions. Simple animals, such as sponges, are made up of only a few types of cell. In more complex animals, cells are organized into tissues, such as muscles and nerves that are necessary for movement. Tissues can form organs, such as the heart, which is used to pump blood around the CIRCULATORY SYSTEM.
All animals with backbones have an internal framework of support, called an endoskeleton. Bony skeletons, such as that of the squirrel, are light to aid movement. When an animal is young, bones in the skeleton can grow in length. Some bones protect vital organs, while limb bones provide anchorage for muscles.
All animals need oxygen to survive. Simple animals exchange gases over the surface of their bodies. Insects, such as caterpillars, have openings along their bodies, called spiracles, through which air passes. Animals with lungs, such as birds and mammals, actively breathe.


Most animals are bilaterally symmetrical. If a penguin were cut in half from head to toe, the two halves would be mirror images of each other. Other animals, such as sea anemones, are radially symmetrical. They have no head or tail and can be cut into identical halves along many lines. Of the two types, animals that are bilaterally symmetrical tend to move more quickly and precisely.


Like all fish, sharks have a backbone, breathe through gills, manoeuvre using fins, and are ectothermic (cold-blooded). A shark’s anatomy also bears the hallmarks of a predatory fish. They have a streamlined, torpedo-shaped body that allows them to cut easily through water to chase prey. They also have powerful jaws and sharp teeth.


Like all arthropods, lobsters have a hard outer casing, called an exoskeleton, made up of plates formed from a substance called chitin. The plates meet at flexible points, such as the leg joints. This exoskeleton provides anchorage for muscles and protection from predators. It also provides support for movement on land and prevents excess water loss.


Animal cells are typically just 0.02 mm (1/1,250 in) across. Although they can be extremely varied, they share common features. Cells are surrounded by a skin called a membrane and contain a jelly-like fluid called cytoplasm. All the processes needed for life, such as producing energy from food, removing waste, and growth take place inside cells.


Inside an animal cell, the cytoplasm contains structures called organelles that have a variety of functions, from storing vital substances to destroying bacteria. The most important organelle is called the nucleus, which carries genetic information, controlling how the cell behaves. Another organelle, the mitochondrion, produces energy from food.


The circulatory system carries blood around an animal’s body, providing nourishment and oxygen to cells. In some animals it is open, in others it is closed. In an open system, blood flows freely around the body. In a closed system, blood is confined to a network of vessels. The circulatory system also helps distribute heat around the body.
Many land animals, such as reptiles, are ectothermic – they rely on the Sun’s heat to raise their body temperature to a level that allows them to be active. Birds and mammals are endothermic – they produce their own heat and maintain a constant body temperature.