Complex sentence: Nautiluses are the only extant cephalopods which have an external shell, although octopuses, cuttlefish and squid (especially "Spirula spirula"), have small internal shells.
Split into simpler sentences: Nautiluses are the only extant cephalopods which have an external shell. Octopuses, cuttlefish and squid (especially "Spirula spirula"), have small internal shells.