Which type of muscle is involuntary and found in the walls of organs?

Smooth muscle is involuntary and is specifically found in the walls of hollow organs such as the intestines, bladder, and blood vessels. This type of muscle functions automatically, meaning it is not under conscious control, which is why it is classified as involuntary. The contractions of smooth muscle help to move substances through these organs, playing a crucial role in processes such as digestion and circulation.

Skeletal muscle, on the other hand, is voluntary and attached to the bones, allowing for movement that can be consciously controlled. Cardiac muscle, while also involuntary, is specific to the heart and is responsible for pumping blood throughout the body. Striated muscle is a term that can refer to both skeletal and cardiac muscle, both of which have a distinct striped appearance, but it does not include smooth muscle. Therefore, smooth muscle is the correct answer as it fits the description given in the question.