The Circulatory System
The brain is the most important organ of the nervous system. The spinal cord and the brain are the main parts of the nervous system but the spinal cord is not considered to be an organ.
The lungs.
lungs
Ovaries
The Kidneys
skin!
Most frogs have lungs, but their are lungless frogs. They can also breath thru their skin.
the skull because it protects the brain
The small intestine is where most of the absorption of nutritients occurs.
organ system
All of it except the appendix. But you have to take care it
Organ systems are most complex and fragile starting with the atoms. The most complex organ systems is the organism. In between are the molecules, cell, tissue, organ, and organ system.