Cephalopods are often regarded as the most intelligent of the invertebrates, and have well developed senses and big brains. Cephalopod neural systems are the most complicated of the invertebrates, with brain-to-body mass ratios that fall between endothermic and ectothermic vertebrates. Most cephalopods rely on vision to detect predators and prey as well as communicate. As a result, cephalopod vision is acute: training trials have revealed that the common octopus can distinguish between object brightness, size, shape, and horizontal or vertical orientation. Cephalopod eyes have the same basic construction as shark eyes, but their construction differs since cephalopods lack a cornea and have an everted retina. Cephalopod eyes are also sensitive to light polarisation planes. Cephalopods have superior vision, statocysts that detect gravity, and a range of chemical sense organs. Octopuses utilise their arms to investigate their surroundings and can use them to detect depth. Cephalopods may change their colours and patterns in milliseconds, whether for signalling or active camouflage, by expanding or contracting their chromatophores. Despite the fact that colour changes appear to be predominantly dependent on vision input, there is evidence that skin cells, specifically chromatophores, may detect light and adjust to light conditions independently of the eyes.