The gladius (plural: gladii), or pen, is a hard internal bodypart found in many cephalopods of the superorder Decapodiformes (particularly squids) and in a single extant member of the Octopodiformes, the vampire squid (Vampyroteuthis infernalis). It 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 (e.g. Actinosepia, Glyphiteuthis, Muensterella, Palaeololigo, Teudopsinia, Teudopsis, and Trachyteuthis), loligosepiids (e.g. Geopeltis, Jeletzkyteuthis, and Loligosepia), and prototeuthids (e.g. Dorateuthis, Paraplesioteuthis, and Plesioteuthis).
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.
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- Young, R.E., M. Vecchione & K.M. Mangold (1999). Cephalopod Gladius Terminology. Tree of Life Web Project.
- Hunt, S. & M. Nixon (1981). A comparative study of protein composition in the chitin-protein complexes of the beak, pen, sucker disc, radula and oesophageal cuticle of cephalopods. Comparative Biochemistry and Physiology Part B: Comparative Biochemistry 68(4): 535–546. doi:10.1016/0305-0491(81)90071-7
- Fuchs, D., T. Engeser & H. Keupp (2007). PDF Acta Palaeontologica Polonica 52(3): 575–589.
- Fuchs, D. (2010). Teudopseina. Tree of Life Web Project.
- (Russian) Bizikov, V.A. (1991). Squid gladius: its use for the study of growth, age, intraspecies structure and evolution (on the example of the family Ommastrephidae). Ph.D. Thesis. Institute of Oceanology, SSSR Academy of Sciences, Moscow. 513 pp. (English abstract)
- Toll, R.B. (1982). The comparative morphology of the gladiolus in the order Teuthoidea (Molluscs: Encephalopathy) in relation to systematic s and phylogeny. PhD.. Thesis. University of Miami, Coral Gables, Florida. 390 pp.
- Toll, R.B. (1998). The gladiolus in Teutonic systematic s. Smithsonian Contributions to Zoology 586(1): 55–67.