Well, your anus contains the anal sphincter which, technically, is an opening of the body. So I would probably say the answer you are looking for is a sphincter muscle.
Otherwise known as circular muscles
The general term for these muscles are sphincters.
The only muscle at the opening of a human body is your tongue.
There are three types of muscles found in the human body. The muscles found in the body are the visceral, cardiac, and skeletal muscles.
Circular Muscles are found at the openings of the body
Smooth muscles are found in many parts of the body. They are mostly found in the digestion and waste disposal systems of the body. The stomach and intestines are smooth muscles.
Involuntary muscles are the muscles in your body that you have no control over. The two main types of involuntary muscles are smooth muscles (which are found in most organs of the body) and cardiac muscle (found in the heart).
What everyone calls the abs are thin muscles of the body.
Sphincter muscles. This muscles contract around orifices keep the body contained.
Smooth Muscles are found in hollow parts of your bodySmooth Muscles are found in hollow parts of your bodySmooth Muscles are found in hollow parts of your bodySmooth Muscles are found in hollow parts of your body
Heart,Lungs
Sphincters (circular muscles)
really almost anywhere in your body
The main organs found in the human body's muscular system are the cardiac muscles, smooth muscles, and skeletal muscles.
Smooth muscles are found in hollow parts of the body. For example they are found in the stomach, intestines, blood vessels, and the bladder. They can contract in all directions.