The octopus is a cephalopod of the order Octopoda that inhabits many diverse regions of the ocean, especially coral reefs. The term may also refer to only those creatures in the genus Octopus. In the larger sense, there are 289 different octopus species, which is over one-third the total number of cephalopod species. Octopuses are characterized by their eight arms, usually with sucker cups on them. These arms are a type of muscular hydrostat. Unlike most other cephalopods, the majority of octopuses â€” those in the suborder most commonly known, Incirrina â€” have almost entirely soft bodies with no internal skeleton. They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squids. A beak, similar in shape to a parrot's beak, is their only hard part. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from morays or other predating fish. The octopuses in the less familiar Cirrina suborder have two fins and an internal shell, making them a little less flexible in being able to squeeze into small spaces. Three defensive mechanisms are typical of octopuses: ink sacs, camouflage, and autotomising limbs. Most octopuses can eject a thick blackish ink in a large cloud to aid in escaping from predators. They also have specialized skin cells both for color changing (chromatophores) and light reflection and refraction (iridophores and leucophores). They use this ability to blend into the environment to hide, as communication with other octopuses, or as a warning: the very poisonous Blue-ringed Octopus becomes bright yellow with blue rings when it is provoked. When under attack, some octopuses can autotomise their limbs, in a similar manner to skinks and other lizards. The crawling arm serves as a distraction to would-be predators; this ability is also used in mating. A few species, such as the Mimic Octopus have a fourth defense mechanism, in that they can combine their highly flexible bodies with their color changing ability to accurately mimic other, more dangerous animals such as lionfish and eels. When octopuses reproduce, they use a specialized arm called a hectocotylus to insert spermatophores (packets of sperm) into the female's mantle cavity. The hectocotylus is usually the third right arm. In some species, the female octopus can keep the sperm alive inside her for weeks until her eggs are mature. After they have been fertilized, the female lays roughly 200,000 eggs (this figure dramatically varies between species). The female hangs these eggs in strings from the ceiling of her lair. After the eggs hatch, the young larval octopuses must spend a period of time drifting in clouds of plankton, where they feed on copepods, larval crabs and larval seastars until they are ready to sink down to the bottom of the ocean, where the cycle repeats itself (in some deeper dwelling species, the young don't go through this period). This is a dangerous time for the larval octopuses, however, as they become part of the plankton cloud and are therefore preyed upon by the many other plankton eaters. Octopuses have a relatively short life span, and some species live for as little as six months. Larger species, such as the North Pacific Giant Octopus, may live for up to five years if they do not reproduce. However, reproduction is a cause of death: males can only live for a few months after mating, and females die shortly after their eggs hatch, for they spend nearly all their time caring for their eggs during the roughly one month period it takes the eggs to hatch, and do not eat during this period. Octopus blood contains the copper-rich protein hemocyanin for transporting oxygen. Less efficient than the iron-rich hemoglobin of vertebrates, the hemocyanin is dissolved in the plasma instead of being bound in red blood cells and gives the blood a blue color.