Animals make up one of the four eukaryotekingdom.  They are all multicellular,since the animal-like unicellular organisms are placed in protoctista.

They differ from plant in beingheterotrophic rather than autotrophic.

They differ from fungi, which are alsomulticellular, 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 theproducts, whereas animal nutrition typically involves ingestion (taking in offood) 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 toanother, and this turn requires a nervous system with sense organs and effectors.Locomotion of larger animals requires muscles and skeleton, which is alsoneeded for support.

In studying animals, we shall be looking atthe evolutionary trends which have led to more and more complex levels oforganisation within their bodies.

One group of animals, the sponges do notform true tissues, but in all other animals tissues are formed. A tissue is agroup of cells, often similar in structure and origin, operating together toperform a specialised function.

Many different tissues can be formed, eachperforming a different function. This is called differentiation or divisionof labour. The same principle operates at the subcellular level, withdifferent organelles showing specialisation for different functions.

Division of labour generally increasesefficiency. Higher levels of organisation than the tissue occur. A number oftissues working together form an organ, such as the stomach, and a group oforgans working together forms a system, such as the digestive system. Thevarious systems together make the organism.

Just as the activity of cells iscoordinated within a tissue, so organs and systems must be coordinated. This isachieved by hormones and a nervous system. The evolutionary development of morecomplex tissues, organs and systems was also accompanied by basic changes inbody plan and, eventually, the need for transport systems within the body,particularly a blood system. Blood is a liquid tissue which is circulated bycontractile vessels or a heart.