What is tissue, an organ and an organ system? What is an example of each?

Hint: The study of animal tissue is called histology. The origin of a tissue may differ in different classes of organisms. The term viscous refers to an organ.

Complete answer:
Tissue is a level of cellular organization between cell and organ. A tissue is the collection and assimilation of cells that have a similar function. The cells forming a tissue must have a similar origin and should carry out a specific function.
There are four types of animal tissues:
1. Connective
2. Muscle
3. Nervous
4. Epithelial
Epithelial tissue consists of cells that form a cover over organ surfaces. For example, the surface of the skin and reproductive tract.
Connective tissues are tissues made up of cells that are separated by an extracellular matrix.
Muscular tissue is made up of muscle cells of the body.
Nervous tissue is made of the cells comprising the CNS and PNS.
Organ is an assimilation of a group of tissues that perform a similar function. The tissues of an organ must perform a specific function. The lungs, liver, pancreas and heart are some examples of organs in the human body. All organisms above the phylum Porifera contain organs. The largest human organ is the skin and the smallest human organ is the pineal body. Some organs are vestigial. These organs have no function in the human body.
An organ system is a collective group of organs performing similar functions in an organism. An example of an organ system is the circulatory system. The circulatory system contains the heart and blood vessels.

Note:
The epithelial tissue is derived from the ectoderm and endoderm.
There are 11 different organ systems in the human body.
The nervous and endocrine organ systems overlap in their constituent organs.