Rectilinear locomotion is a mode of locomotion

Animal locomotion

Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...

 most often associated with snake

Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s, particularly heavy-bodied species like terrestrial pythons and boas, although most snakes are capable of it. Unlike all other modes of snake locomotion, in which the snake bends its body, in rectilinear locomotion, the snake flexes its body only when turning.

Rectilinear locomotion relies upon two opposing muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s, the costcutaneous inferior and superior, which are present on every rib

Rib

In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

 and connect the rib to the skin

Skin

-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

. The ribs themselves do not move, only the skin. First, the costcutaneous superior lifts a section of the snake's belly from the ground and places it ahead of its former position. Then, the costcutaneous inferior pulls backwards while the belly scales are on the ground, propelling the snake forwards. These sections of contact propagate posteriorly, resulting in continuous motion.

This method of locomotion is extremely slow, but is also almost noiseless and very hard to detect, making it the mode of choice for many species when stalking prey.