The fin cartilage in decapodiforms is a small, flat cartilage at the base of the fin that the fin muscles attach to. The cartilage in turn attaches to the shell sac which tightly envelops the shell. In octopodiforms, however, the fin cartilage is much more extensive, extending well down the fin axis laterally but maintains attachment to the shell sac and shell proximally. Although the cartilage is flexible, it presumably aids in maintaining the somewhat rigid form of the fin. The fins of octopodiforms are very differently shaped from those of decapodiforms and have a different swimming action.
Figure. Dorsal view of a fin, the shell and the shell cartilage of the cirrate octopod, Cirroctopus glacialis. An isolated fin (left) and an isolated fin cartilage (right) attach to the shell via the shell sac (the shell sac is a thin, tough membrane tightly covering the shell and is not apparent in the drawing). Drawing modified from Young and Vecchione and Young (1996).
