- Shell located internally.
- Muscular mantle for propulsion and respiration.
- Brachial crown consisting of ten arms.
Figure. The diagramatic illustrations show the arm-crown modifications found in the major groups of coleoid cephalopods. The Belemnoidea represent the primitive condition with 10 equal arms. In the Vampyromorpha arms II are modified and in the only living representative, Vampyroteuthis, these are a pair of slender, sensory filaments that arise from the aboral surface of the web. In the Octopoda arm pair II is lost leaving 8 arms, and in the Decapodiformes, arms IV are modified into tentacles with suckers generally confined to distal clubs.
- Fins present*
- Chromatophores present*
- Ink sac present.
- Eyes with lenses.
* presence not established in Belemnoidea.
Many of these characters have been secondarily lost in various taxa.
Arm pairs are counted from dorsal to ventral as indicated in the illustration above. However, the numbering of arms in taxonomic descriptions leads to some ambiguity. In the Decapodiformes, the fourth of five pairs of arms has been modified as tentacles, leaving four pairs of functional arms. As a result, the ventral arms are the true fifth pair but functionally are the fourth pair. Similarly, the second pair of arms is modified in vampyroteuthids and lost in octopods causing a difference between true (i.e., homologous) and functional arm-pair numbering. True arm-pair numbering is used in the above illustration while functional numbering is used in descriptions of taxa.
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