Fig. 2. The scoring system for tentacle extension response (TER) in chambered nautilus. TER was graded every 5 s from a minimum score of 0 to a maximum score of 3. Each level corresponds to a range of percentage extension relative to the length of the animal's hood. Zero is recorded when all tentacles are retracted into their sheaths. A score of 1 corresponds to an extension of <33% of the hood length. A score of 2 corresponds to extension between 34% and 66%, and 3 is given when tentacles are extended beyond 67% of hood length.