Octopus
There are 289 different octopus species, which is over one-third the total number of cephalopod species. Octopuses are characterized by their eight arms (not tentacles), usually bearing suction cups. These arms are a type of a muscular hydrostat. Unlike most other cephalopods, the majority of octopuses 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 parrot like beak is their only hard part. This beak enables them to squeeze through very narrow slits between underwater rocks which aids them in escaping from their moray predators and other predatory fish.
Octopuses have a relatively short life span, and some species live for as little as six months. Octopuses have three hearts. Two pump blood through each of the two gills, while the third pumps blood through the body. Octopuses draw water into their mantle cavity where it passes through its gills. As mollusks, octopuses have gills that are finely divided and vascularized outgrowths of either the outer or the inner body surface.
Three defensive mechanisms are typical of octopuses. The first being that many octopuses can eject a thick blackish ink in a large cloud to help them escape from predators. Second, their amazing ability to camouflage themselves. Their camouflage is a combination body positioning and skin coloring. Specialized skin cells which can change the pigment, opacity, and reflectiveness of the epidermis. Chromatophores may turn yellow, orange, red, brown, or black; most species have three of these colors, while some have two or four. Other color-changing cells are iridophores (pink, yellow, green,
blue, or silver),reflector cells (blue and green), and leucophores (white). This color-changing ability can also be used to communicate with or warn other octopuses. The very poisonous blue-ringed octopus becomes bright yellow with blue rings when it is provoked. Their third defense mechanism is an ability to autotomise their limbs. 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 (see photo below) have a fourth defense mechanism. They can combine their highly flexible bodies with their color changing ability to accurately mimic other, more dangerous animals such as lionfish or eels. They have also been observed changing the texture of their mantle in order to achieve a greater camouflage. The 
mantle can take on the spiky appearance of seaweed, or the scraggly, bumpy texture of a rock, among other disguises.
Octopuses are highly intelligent, with their intelligence supposedly comparable to that of the average cat. Maze and problem-solving experiments show that they have both short- and long-term memory, although their short lifespans limit the amount they can ultimately learn. In laboratory experiments, octopuses can be readily trained to distinguish between different shapes and patterns. They are able to open jars after learning from observation. Octopuses have also been observed in what may be described as play; repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them. Octopuses often break out of their aquariums and sometimes into others in search of food. They have even boarded fishing ships and opened holds to eat crabs. They are even listed as experimental animals on which surgery may not be performed without anesthesia.
Octopuses have keen eyesight although they do not appear to have color vision. Attached to the brain are two special organs, called statocysts, which allow the octopus to sense the orientation of its body relative to horizontal. An autonomic response keeps the octopus's eyes oriented so that the pupil slit is always horizontal.
Octopuses also have an excellent sense of touch. The octopus's suckers are equipped with chemoreceptors so that the octopus can taste what it is touching. The arms contain tension sensors so that the octopus knows whether its arms are stretched out. The tension receptors are not sufficient for the octopus brain to determine the position of the octopus's body or arms. As a result, the octopus , does not form a mental image of the overall shape of the object it is handling. It can detect local texture variations, but cannot integrate the information into a larger picture. There is no neurological path for the brain to receive feedback about just how its command was executed by the arms; the only way it knows just what motions were made is by observing the arms visually.
Octopuses move about by crawling or swimming. Their fastest movements only occur when provoked by hunger or if in danger. Because the rate at which oxygen is transported to the blood is an estimated 4% in octopuses, they have a high disadvantage in the wild because of poor stamina.
Encounters
The octopus inhabits many diverse regions of the ocean, especially coral reefs.
Reproduction
Reproduction is one reason for the octopus’s short lifespan: males can only live for a few months after mating, and females die shortly after their eggs hatch, for they neglect to eat during the (roughly) one month period spent taking care of their unhatched eggs.
When octopuses reproduce, males use a specialized arm called a hectocotylus to insert packets of sperm into the female's mantle cavity. After they have been fertilized, the female lays roughly 200,000 eggs (this figure dramatically varies between families, genera, species and also individuals). The female hangs these eggs in strings from the ceiling of her lair, or individually attached to the substratum depending on the species. 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 do not go through this period. This is a dangerous time for the larval octopuses; as they become part of the plankton cloud they are vulnerable to many plankton eaters.
Captivity
Although octopuses can be hard to keep in captivity, some people keep them as pets anyway. Octopuses often escape even from supposedly secure tanks, due to their intelligence and problem solving skills. Octopuses are also quite strong for their size. Some have been known to open the covers of their aquariums and survive for a time in the air in order to get to a nearby feeder tank and gorge themselves on the fish there. They have also been known to catch and kill some species of sharks.
CLASS CEPHALOPODA
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- Subclass Nautiloidea: nautilus
- Subclass Coleoidea: squid, octopus, cuttlefish
- Superorder Octopodiformes
- Superorder Decapodiformes
- Order Spirulida: Ram's Horn Squid
- Order Sepiida : cuttlefish
- Family Sepiadariidae
- Family Sepiidae
- Order Sepiolida: bobtail squid
- Order Teuthida: squid