The octopus is bilaterally symmetrical along its dorso-ventral axis; the head and foot are at one end of an elongated body and function as the anterior (front) of the animal. Most of the body is made of soft tissue allowing it to lengthen, contract, and contort itself. The octopus can squeeze through tiny gaps; even the larger species can pass through an opening close to 2.5 cm (1 in) in diameter.
The head includes the mouth and brain.
The eyes of the octopus are large and are at the top of the head. They are similar in structure to those of a fish and are enclosed in a cartilaginous capsule fused to the cranium. The cornea is formed from a translucent epidermal layer and the slit-shaped pupil forms a hole in the iris and lies just behind. The lens is suspended behind the pupil and photoreceptive retinal cells cover the back of the eye. The pupil can be adjusted in size and a retinal pigment screens incident light in bright conditions.
Siphons in molluscs are tube-like structures in which water flows. The water flow is used for one or more purposes such as locomotion, feeding, respiration, and reproduction. The siphon is part of the mantle, and the water flow is directed from the mantle cavity.
Barring a few exceptions, octopuses have eight arms and no tentacles, while squid and cuttlefish have eight arms (or two "legs" and six "arms") and two tentacles. The arms can be described based on side and sequence position (such as L1, R1, L2, R2) and divided into four pairs. The two rear appendages are generally used to walk on the sea floor, while the other six are used to forage for food; hence some biologists refer to the animals as having six "arms" and two "legs". Eledone cirrhosa is one of the octopus species with only one row of suckers.
A sucker is usually circular and bowl-like and has two distinct parts: an outer shallow cavity called an infundibulum and a central hollow cavity called an acetabulum. Both of these structures are thick muscles, and are covered with a chitinous cuticle to make a protective surface. Suckers are used for grasping substratum, catching prey and for locomotion.
The mouth is the start of the digestive system. The mouth is part of the buccal mass and has a chitinous beak, a pharynx, radula and salivary glands.