What is an organ of a multicellular animal? What organ systems are distinguished in animals? What functions do they perform?

An organ is a part of a multicellular organism, formed by various tissues and acting as a whole when performing a complex function.
In the body of animals, the following organ systems usually interact: musculoskeletal, digestive, respiratory, excretory, circulatory, reproductive, nervous and endocrine.
The musculoskeletal system consists of the skeleton and the muscles that are attached to it. This system performs supporting, motor and protective functions in the animal’s body.
The digestive system consists of organs that provide chopping and digestion of food, absorption of nutrients and removal of undigested residues from the body.
The excretory system serves to remove excess water and harmful metabolic products from the body. The circulatory system provides the movement of blood throughout the body.