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.[2][3] It is usually roughly arch- or saddle-shaped and is rather soft, being similar in consistency to cartilage.[4] Each of the eight extant cirrate genera is characterised by a distinct shell morphology:[5]
The comparatively simple shells of Opisthoteuthidae and Stauroteuthidae are thought to approximate the ancestral shape, with those of Cirroteuthidae being more derived.[8] 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.[8] The paired, rod-shaped stylets of incirrates are evolutionarily derived from the lateral wings and horns of the cirrate shell.[8]
|
Cephalopod anatomy
| ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Shell |
|
| ||||||||||||
| Mantle & funnel |
| |||||||||||||
| Head & limbs |
| |||||||||||||
| General |
| |||||||||||||
Developmental stages: Spawn → Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult • Egg fossils • Protoconch (embryonic shell) | ||||||||||||||
