Epithelial tissue covers the entire surface of many organisms. which term describes the structure directly below tissues in the hierarchical organization of life?
a) atoms
b) cells
c) molecules
d) organs

Organs or organ systems are directly below the hierarchical organization of life. For an organism to act or function properly organ systems are needed to exist. The following are examples of organ systems that are needed by an organism:

1. Nervous System 
2. Digestive System
3. Reproductive System
4. Respiratory System
5. Muscular System
6. Skeletal System
7. Endocrine System
8. Immune System
9. Excretory System
10. Circulatory System
11. Integumentary System