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.
The cirrate shell is quite unlike that of any other living cephalopod group and has its own dedicated set of descriptive terms. It is usually roughly arch- or saddle-shaped and is rather soft, being similar in consistency to cartilage. Each of the eight extant cirrate genera is characterised by a distinct shell morphology:
- Cirroctopus — V-shaped, lateral wings tapering to fine points
- Cryptoteuthis — U-shaped, each lateral wing ending in broad lobe with pointed projection
- Grimpoteuthis — U-shaped, lateral wings ending bluntly
- Luteuthis — W-shaped
- Opisthoteuthis — U-shaped, lateral wings usually tapering to fine points but termination complex in certain species
- Stauroteuthis — U-shaped
The comparatively simple shells of Opisthoteuthidae and Stauroteuthidae are thought to approximate the ancestral shape, with those of Cirroteuthidae being more derived. The shell of Cirroctopus appears transitional in form between those of incirrate octopuses and other cirrates, and resembles the reduced shell of the Late Cretaceous Palaeoctopus newboldi. The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell.
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