Cirrate octopuses lack the ink sac. , Ancient seafaring people were aware of the octopus, as evidenced by certain artworks and designs. They are known as "soft-bodied" cephalopods, lacking the external shell of most molluscs and other cephalopods like the nautiloids and the extinct Ammonoidea.  The mantle cavity has muscular walls and contains the gills; it is connected to the exterior by a funnel or siphon. When the octopus is approached, it may extend an arm to investigate. As progress is made, other arms move ahead to repeat these actions and the original suckers detach.  Octopus species that produce larger eggs – including the southern blue-ringed, Caribbean reef, California two-spot, Eledone moschata and deep sea octopuses – do not have a paralarval stage, but hatch as benthic animals similar to the adults. One such species is the giant Pacific octopus, in which courtship is accompanied, especially in the male, by changes in skin texture and colour. Octopus is an animal that is deaf.  The arms and sometimes other body parts are prepared in various ways, often varying by species or geography. , When octopuses reproduce, the male uses a specialised arm called a hectocotylus to transfer spermatophores (packets of sperm) from the terminal organ of the reproductive tract (the cephalopod "penis") into the female's mantle cavity. In an alternative method of swimming, some species flatten themselves dorso-ventrally, and swim with the arms held out sideways, and this may provide lift and be faster than normal swimming.  Ian Fleming's 1966 short story collection Octopussy and The Living Daylights, and the 1983 James Bond film were partly inspired by Hugo's book. Octopuses have an innate immune system, and the haemocytes respond to infection by phagocytosis, encapsulation, infiltration or cytotoxic activities to destroy or isolate the pathogens. The head of an Octopus is very large. The male may cling to the top or side of the female or position himself beside her. , Most species are solitary when not mating, though a few are known to occur in high densities and with frequent interactions, signaling, mate defending and eviction of individuals from dens. There is some speculation that he may first use his hectocotylus to remove any spermatophore or sperm already present in the female. , The arms contain tension sensors so the octopus knows whether its arms are stretched out, but this is not sufficient for the brain to determine the position of the octopus's body or arms.  It used to be thought that the hole was drilled by the radula, but it has now been shown that minute teeth at the tip of the salivary papilla are involved, and an enzyme in the toxic saliva is used to dissolve the calcium carbonate of the shell. As it moves the eight arms move along behind it. That is what they inject into their prey to immobilize it. The testis in males and the ovary in females bulges into the gonocoel and the gametes are released here. All octopuses are venomous, but only the blue-ringed octopuses are known to be deadly to humans.  The lamella structure of the gills allows for a high oxygen uptake, up to 65% in water at 20 °C (68 °F).  The hectocotylus in benthic octopuses is usually the third right arm, which has a spoon-shaped depression and modified suckers near the tip.  The larger Pacific striped octopus however is social, living in groups of up to 40 individuals that share dens. , Octopuses are highly intelligent; the extent of their intelligence and learning capability are not well defined.  The largest specimen of this species to be scientifically documented was an animal with a live mass of 71 kg (156.5 lb). The file-like organ is covered with tiny, sharp teeth that are replaced when they wear down, much as sharks regrow teeth. , Attached to the brain are two special organs called statocysts (sac-like structures containing a mineralised mass and sensitive hairs), that allow the octopus to sense the orientation of its body. Octopus Reproduction Read Or Listen To The Article Click Here. Two spermatophores are transferred in this way; these are about one metre (yard) long, and the empty ends may protrude from the female's mantle.  In addition, the internal shell of incirrates is either present as a pair of stylets or absent altogether. Octopuses have only six arms, the other two are actually legs! In some species, the mantle can take on the spiky appearance of algae; in others, skin anatomy is limited to relatively uniform shades of one colour with limited skin texture. From there it is transferred to the gastrointestinal tract, which is mostly suspended from the roof of the mantle cavity by numerous membranes.  The smallest species is Octopus wolfi, which is around 2.5 cm (1 in) and weighs less than 1 g (0.035 oz). The arms later migrate upwards, coming to form a ring around the funnel and mouth.  Octopus reproductive organs mature due to the hormonal influence of the optic gland but result in the inactivation of their digestive glands, typically causing the octopus to die from starvation. As juveniles, common octopuses inhabit shallow tide pools.  Unlike vertebrates, the complex motor skills of octopuses are not organised in their brain via an internal somatotopic map of its body, instead using a nonsomatotopic system unique to large-brained invertebrates. Basal species, the Cirrina, have stout gelatinous bodies with webbing that reaches near the tip of their arms, and two large fins above the eyes, supported by an internal shell.  The cirrate species are often free-swimming and live in deep-water habitats.  They have even boarded fishing boats and opened holds to eat crabs.  Octopuses have also been observed in what has been described as play: repeatedly releasing bottles or toys into a circular current in their aquariums and then catching them. , The skin consists of a thin outer epidermis with mucous cells and sensory cells, and a connective tissue dermis consisting largely of collagen fibres and various cells allowing colour change. The third one is responsible for pumping the blood through the rest of the body. The scientific Latin term octopus was derived from Ancient Greek ὀκτώπους, a compound form of ὀκτώ (oktō, "eight") and πούς (pous, "foot"), itself a variant form of ὀκτάπους, a word used for example by Alexander of Tralles (c. 525–605) for the common octopus. Well known for a very large head and eight arms, it is able to move around the water with speed and grace. I. Biomechanics of the octopus reaching movement", "Color Discrimination Conditioning in Two Octopus, "Octopus vision, it's in the eye (or skin) of the beholder", "Study proposes explanation for how cephalopods see colour, despite black and white vision", "Odd pupils let 'colorblind' octopuses see colors", "Cephalopod Ink: Production, Chemistry, Functions and Applications", "Giant Pacific Octopus (Enteroctopus dofleini) Care Manual", "Absurd Creature of the Week: The Beautiful Octopus Whose Sex Is All About Dismemberment", "Octopuses were thought to be solitary until a social species turned up", "Octopus Senescence: The Beginning of the End", Journal of Applied Animal Welfare Science, "First in situ observation of Cephalopoda at hadal depths (Octopoda: Opisthoteuthidae: Grimpoteuthis sp. Lb ) a food conversion efficiency greater than that of chickens, making octopus aquaculture possibility... With its arms, the orifice between the two structures is sealed failure to. 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