Cirrate shell

Cirrate octopuses possess a well-developed internal shell that supports their muscular swimming fins.

This is in contrast to the more familiar, finless, incirrate octopuses, in which the shell remnant is either present as a pair of stylets or absent altogether.

[1] The cirrate shell is quite unlike that of any other living cephalopod group and has its own dedicated set of descriptive terms.

[4] Each of the eight extant cirrate genera is characterised by a distinct shell morphology outlined below[5] (below taxonomy updated per WoRMS): The comparatively simple shells of Opisthoteuthidae and Stauroteuthidae are thought to approximate the ancestral shape, with those of Cirroteuthidae being more derived.

[9] The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell.