Animals make up one of the four eukaryote kingdom. They are all multicellular, since the animal-like unicellular organisms are placed in protoctista.
They differ from plant in being heterotrophic rather than autotrophic.
They differ from fungi, which are also multicellular, heterotrophic eukaryotes, in the way they obtain their food. Fungi can be described as absorptive and animals ingestive. Fungi digest food outside their bodies and absorb the products, whereas animal nutrition typically involves ingestion (taking in of food) followed by digestion inside the body. Any undigested food is egested (got rid of outside the body).
A number of feeding habits have developed, including carnivorous, herbivorous, omnivorous and parasitic modes of life. Whereas fungi grow on their food, animals often have to seek it. If they do, this requires locomotion, the ability of the animal to move from one place to another, and this turn requires a nervous system with sense organs and effectors. Locomotion of larger animals requires muscles and skeleton, which is also needed for support.
In studying animals, we shall be looking at the evolutionary trends which have led to more and more complex levels of organisation within their bodies.
One group of animals, the sponges do not form true tissues, but in all other animals tissues are formed. A tissue is a group of cells, often similar in structure and origin, operating together to perform a specialised function.
Many different tissues can be formed, each performing a different function. This is called differentiation or division of labour. The same principle operates at the subcellular level, with different organelles showing specialisation for different functions.
Division of labour generally increases efficiency. Higher levels of organisation than the tissue occur. A number of tissues working together form an organ, such as the stomach, and a group of organs working together forms a system, such as the digestive system. The various systems together make the organism.
Just as the activity of cells is coordinated within a tissue, so organs and systems must be coordinated. This is achieved by hormones and a nervous system. The evolutionary development of more complex tissues, organs and systems was also accompanied by basic changes in body plan and, eventually, the need for transport systems within the body, particularly a blood system. Blood is a liquid tissue which is circulated by contractile vessels or a heart.