Muscular System of the Human Body
Invertebrates have muscles attached to exoskeletons. Insects are examples. In animals without exoskeletons, such as squids, muscles are not affixed.  Vertebrates, animals with backbones, have three types of muscle tissue -- cardiac, smooth, and skeletal. Muscle tissue in a healthy average human accounts for approximately 50 percent of total body weight.
Cardiac muscle is found only in the heart, thus its name. Smooth muscle is found in the intestinal wall, in the stomach and in walls of blood vessels. Because smooth muscle is found in internal organs of the body, or viscera, it is sometimes called "visceral muscle." Smooth muscle is also found in the eye. Skeletal muscles are attached to the bones of the human skeleton by tendons.