The yolk is gradually absorbed as the embryo develops. They can extend and contract, twist to left or right, bend at any place in any direction or be held rigid. They can use navigation skills to return to a den without having to retrace their outward route. Muscles in the skin change the texture of the mantle to achieve greater camouflage. Young octopuses learn nothing from their parents, as adults provide no parental care beyond tending to their eggs until the young octopuses hatch. The urine is first formed in the pericardial cavity, and is modified by excretion, chiefly of ammonia, and selective absorption from the renal appendages, as it is passed along the associated duct and through the nephridiopore into the mantle cavity. This makes the blood very viscous and it requires considerable pressure to pump it round the body; octopuses' blood pressures can exceed 75 mmHg. The foot has evolved into a set of flexible, prehensile appendages , known as "arms", that surround the mouth and are attached to each other near their base by a webbed structure.
Fleshy papillae or cirri are found along the bottom of the arms, and the eyes are more developed. Its nervous system allows the arms to move with some autonomy. Colour vision appears to vary from species to species, for example being present in O. They can extend and contract, twist to left or right, bend at any place in any direction or be held rigid. It is not known precisely what contribution learning makes to adult octopus behaviour. The blood vessels consist of arteries, capillaries and veins and are lined with a cellular endothelium which is quite unlike that of most other invertebrates. They are similar in structure to those of a fish and are enclosed in a cartilaginous capsule fused to the cranium. Octopuses usually inject crustaceans like crabs with a paralysing saliva then dismember them with their beaks. The giant Pacific octopus Enteroctopus dofleini is often cited as the largest known octopus species. The infundibulum provides adhesion while the acetabulum remains free, and muscle contractions allow for attachment and detachment. The giant Pacific octopus , one of the two largest species of octopus, may live for as much as five years. 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. This moves the body slowly. Octopus arms do not become tangled or stuck to each other because the sensors recognise octopus skin and prevent self-attachment. In this she broods the young, and it also serves as a buoyancy aid allowing her to adjust her depth. The tract consists of a crop , where the food is stored; a stomach, where food is ground down; a caecum where the now sludgy food is sorted into fluids and particles and which plays an important role in absorption; the digestive gland , where liver cells break down and absorb the fluid and become "brown bodies"; and the intestine, where the accumulated waste is turned into faecal ropes by secretions and blown out of the funnel via the rectum. During gastrulation , the margins of this grow down and surround the yolk, forming a yolk sac, which eventually forms part of the gut. The gland may be triggered by environmental conditions such as temperature, light and nutrition, which thus control the timing of reproduction and lifespan. The octopus can squeeze through tiny gaps; even the larger species can pass through an opening close to 2. Sometimes the octopus catches more prey than it can eat, and the den is often surrounded by a midden of dead and uneaten food items. The male may cling to the top or side of the female or position himself beside her. He picks up a spermatophore from his spermatophoric sac with the hectocotylus, inserts it into the female's mantle cavity, and deposits it in the correct location for the species, which in the giant Pacific octopus is the opening of the oviduct. The creature may make a jet-propelled pounce on prey and pull it towards the mouth with its arms, the suckers restraining it. Octopuses have three hearts; a systemic heart that circulates blood round the body and two branchial hearts that pump it through each of the two gills. Octopuses are not territorial but generally remain in a home range; they may leave the area in search of food.
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