Cephalopod intelligence is a measure of the cognitive ability of the cephalopod class of molluscs. Spirula spirula is a species of deep-water squid-like cephalopod mollusk. Its body resembles a massive, cuttlefish-like cephalopod. The heaviest known cephalopod, and the largest living invertebrate, is the colossal squid. Octopus and arrow squid make up most of their cephalopod diet. Active camouflage is used in several groups of animals including cephalopod molluscs, fish, and reptiles. Before death cuttlefish go through senescence when the cephalopod essentially deteriorates, or rots in place. An orthocone is an unusually long straight shell of a nautiloid cephalopod. The taxonomy of the giant squid, as with many cephalopod genera, has long been debated. Superficial similarities between cephalopod and vertebrate eyes are thought to be examples of convergent evolution. "Spirula spirula" is a deep water squid-like cephalopod. Putative cephalopod egg fossils have also been documented. There has been debate on the embryological origin of cephalopod appendages. Cephalopod appendages surround the mouth, so logically they could be derived from embryonic head tissues. The cephalopod beak resembles that of a parrot.