They belong to the class Cephalopoda, which also includes squid, octopuses, and nautiluses. Nautiluses and the more familiar incirrate octopuses lack swimming fins. "N. p. pompilius" is by far the most common and widespread of all nautiluses. The registry only covers the shells of nautiluses and "Spirula" and the eggcases of "Argonauta". Nautiluses generally avoid water temperatures above . However the long-term memory capability of nautiluses was much shorter than that of other cephalopods. Nautiluses usually inhabit depths of several hundred metres. However, the long-term memory capability of nautiluses is much shorter than that of Coleoidea. Both octopuses and nautiluses are capable of vertebrate-like spatial learning. The limbs of nautiluses, which number around 90 and lack suckers altogether, are called tentacles. Mainly scavengers, chambered nautiluses have been described as eating "anything that smells". Nautiluses are the only extant cephalopods with a true external shell. They are also called paper nautiluses, referring to the paper-thin eggcase that females secrete. Nautiluses are the only extant cephalopods which have an external shell.