Unstriped muscles are smooth muscles that contract without conscious control (involuntary), having the form of thin layers or sheets. They are made up of spindle-shaped, unstriated cells with single nuclei and are found in the walls of the internal organs, such as the stomach, bladder, intestine and blood vessels (except for the heart). The muscles of the iris are the smooth muscles in mammals and amphibians but are the striated muscle in reptiles (including birds).
Thus, the correct answer is option C.