Squid gladius, showing measurement of rachis and vane

The gladius (plural: ''gladii''), or pen, is a hard internal bodypart found in many

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s of the superorder

Decapodiformes Decapodiformes is a superorder of Cephalopoda comprising all cephalopod species with ten limbs, specifically eight short arms and two long tentacles. It is hypothesized that the ancestral coleoid had five identical pairs of limbs, and that one ...

(particularly

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...

s) and in a single

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

member of the

Octopodiformes Octopodiformes is a superorder of the subclass Coleoidea, comprising the octopuses and the vampire squid. All living members of Octopodiformes have eight arms, either lacking the two tentacles of squid (as is the case in octopuses) or modifyin ...

, the

vampire squid The vampire squid (''Vampyroteuthis infernalis'', lit. 'vampire squid from hell') is a small cephalopod found throughout temperate and tropical oceans in #Habitat and adaptations, extreme deep sea conditions. The vampire squid uses its Biolumines ...

(''Vampyroteuthis infernalis''). It is so named for its superficial resemblance to the

Roman Roman or Romans most often refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a letter ...

short sword The English language terminology used in the classification of swords is imprecise and has varied widely over time. There is no historical dictionary for the universal names, classification or terminology of swords; a sword was simply a double ...

of the same name, and is a vestige of the ancestral

mollusc shell The mollusc (or molluskOften spelled mollusk shell in the USA; the spelling "mollusc" are preferred by ) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, wh ...

, which was external. The gladius is located

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...

ly within the mantle and usually extends for its entire length. Composed primarily of

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

, it lies within the shell sac, which is responsible for its secretion. Trachyteuthis hastiformis 01

Gladii are known from a number of extinct cephalopod groups, including teudopseids (''e.g.'' '' Actinosepia'', '' Glyphiteuthis'', '' Muensterella'', '' Palaeololigo'', '' Teudopsinia'', ''

Teudopsis ''Teudopsis'' is a genus of the coleoid cephalopod family Teudopsidae, known only from Lower Jurassic gladii. It has been reported from Alberta, Canada and Europe. The life appearance of Teudopsis is probably best inferred by its close relativ ...

'', and ''

Trachyteuthis ''Trachyteuthis'' is a genus of fossil cephalopod, comprising five species: ''T. hastiformis'', ''T. latipinnis'', ''T. nusplingensis'', ''T. teudopsiformis'', ''T. covacevichi'' and ''T. chilensis''. Taxonomy The taxonomic placement of ''Tr ...

''), loligosepiids (''e.g.'' '' Geopeltis'', '' Jeletzkyteuthis'', and '' Loligosepia''), and prototeuthids (''e.g.'' ''

Dorateuthis ''Dorateuthis'' is a genus of cephalopod with a gladius and soft-part anatomy preserved. Fossils of ''D. syriaca'' are found in Upper Santonian-aged shale of Late Cretaceous Lebanon Lebanon ( , ar, لُبْنَان, translit=lubnān, ) ...

'', '' Paraplesioteuthis'', and ''

Plesioteuthis ''Plesioteuthis'' is an extinct genus of squids, belonging to the family Plesioteuthididae. This genus was first described in 1859, and its fossils were found in the well-known Solnhofen field in Germany. Species Species within this genus inclu ...

'').

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. Chiroteuthis veranyi immature

Variability

See also

References

Further reading