sentence: Nautiluses are the only cephalopods who have an external shell.
sentences_after: This is an ancestral or basal, feature. The animal can withdraw completely into its shell. It can the close the opening with a leathery hood formed from two specially folded tentacles. It is pressure resistant, but will implode at a depth of about 800 m. The innermost portion of the shell is pearlescent, blue-gray.
concept_name: nautilus
headings: Nautilus The shell
categories: Cephalopods Living fossils