The gladius, found in many decapodiforms and Vampyroteuthis, is an internal remnant of the originally external shell of cephalopods. Other cephalopods have other types of shell remnants. The gladius lies within a sac, the shell sac, which secretes it and to which various muscles attach. The gladius has a variety of shapes and thicknesses, is composed mostly of chitin and is located in the dorsal midline and generally extends the full length of the mantle.
Conus field - The lateral portion of the gladius that is continuous posteriorly with the primary conus or its remnant and laterally with the edges of the vanes. The conus field is differentiated from the vanes by longitudinal lines and marked changes in the orientation of the growth lines. A conus field is generally lacking if the gladius lacks a primary conus.
Figure. Dorsal view of the gladius of Abraliopsis sp. A, modified from Young and Harman, 1998.
Primary conus - The cup or cone-shaped posterior end of the gladius of some cephalopods.
Rhachis - Central axis of the gladius that typically extends the full length of the gladius and sometimes called the median field. The anterior end of the gladius consists of the rhachis only and the rhachis here is known as the free rhachis. The rhachis sometimes contains a thickened central ridge or keel.
Figure. Ventral view of the gladius of Doryteuthis pealeii from Toll, 1998.
Rostrum - The solid extension of the gladius, triangular or conical, located posterodorsally to the conus in the Onychoteuthidae, Ancistrocheiridae, and posterior to the conus in the Lampadioteuthinae and Vampyroteuthidae. These are the only taxa that possess a rostrum.
Figure. Ventral view of the gladius of Lampadioteuthis megaleia from Toll, 1998.
Secondary conus - A conus formed by the ventral infolding and fusion of the posterolateral edges of the vanes. The ventral line of fusion where the vanes join one another is usually apparent and helps distinguish the secondary from the primary conus. The secondary conus may be rather short or over half the gladius length.
Vane - The two lateral "wings" of the gladius.