Hierarchy map for excretory system in an animal:

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The stomach is found in only one organ system, but skin is found in more than one organ system

Many specialized cells work together to form a tissue and many specialized tissues work together to form an organ and many organs work in association with each other to form an organ system. The organ system is therefore more sophisticated and complex than specialized cells because it is playing a specific role in system in coordination with other organs.

The organ system is therefore sophisticated and complex than specialized cells because it is playing a specific role in system in coordination with other organs.

The heart pumps blood to all parts of the body by the help of the circulatory system.

The brain transmits signal to all parts of the body by the help of the nervous system.

The lungs extract oxygen from air for all parts of the body by the help of the respiratory system.

The stomach digests food for the body by the help of the digestive system.

The kidneys and skin collect waste from all parts of the body and excrete them by the help of the excretory system.

Organ systems and their specific roles are pretty much standard in most animals because like human being bodies, animal bodies have the same basic needs and requirements and therefore similar systems are in place for meeting those requirements. For example all animals like human beings respire, eat, defecate, excrete, move etc.

Like human being bodies, animal bodies have the same basic needs and requirements and therefore similar systems are in place for meeting those requirements.

Single celled organisms are composed of one cell in which all its cellular activities are controlled by different units of the cell and therefore no hierarchy of organization is required.