Gladius (cephalopod)
The gladius, or pen, is a hard internal bodypart found in many cephalopods of the superorder Decapodiformes and in a single extant member of the Octopodiformes, the vampire squid. It is so named for its superficial resemblance to the Roman short sword of the same name, and is a vestige of the ancestral mollusc shell, which was external. The gladius is located dorsally within the mantle and usually extends for its entire length. Composed primarily of chitin, it lies within the shell sac, which is responsible for its secretion.
Gladii are known from a number of extinct cephalopod groups, including teudopseids, loligosepiids, and prototeuthids.
Variability
Gladii are shaped in many distinctive ways and vary considerably between species, though are often like a feather or leaf. Some examples are shown below.| Shape of gladius | Species | Family |
| Ancistroteuthis lichtensteini | Onychoteuthidae | |
| Architeuthis sp. | Architeuthidae | |
| Bathyteuthis abyssicola | Bathyteuthidae | |
| Histioteuthis bonnellii | Histioteuthidae | |
| Histioteuthis reversa | Histioteuthidae | |
| Illex illecebrosus | Ommastrephidae | |
| Lepidoteuthis grimaldii | Lepidoteuthidae | |
| Doryteuthis pealeii | Loliginidae | |
| Loliolus sumatrensis | Loliginidae | |
| Lolliguncula brevis | Loliginidae | |
| Mastigoteuthis agassizii | Mastigoteuthidae | |
| Onykia ingens | Onychoteuthidae | |
| Pholidoteuthis massyae | Pholidoteuthidae | |
| Sepioteuthis lessoniana | Loliginidae | |
| Taningia danae | Octopoteuthidae | |
| Taonius borealis | Cranchiidae | |
| Teuthowenia megalops | Cranchiidae | |
| Uroteuthis duvauceli | Loliginidae |
chiroteuthids are unusual in that they possess a greatly elongated gladius extending well beyond the fins; this supports a long, trailing tail-like structure.