Fig. 19. Inferences about the context-dependent function of the radular stalk, 17 and 15 from odontophore kinematics. All illustrations are in orthographic projection, with the radula/odontophore rotated such that the radular stalk is vertical. The top row (A—C) shows transparent antero-posterior views of the model from fully open to just after radular closure. The middle row (D—F) shows antero-posterior views (i.e. through the jaws) of the radula/odontophore with the inferred locations of its constituent muscles indicated schematically. The bottom row (G—I) shows postero-anterior views (i.e. through the esophagus) of the radula/odontophore with the inferred locations of its constituent muscles indicated schematically. The first column shows the radula/odontophore before peak protraction (t4 period) from frame 22 of sequence 7732-S3. The inferred borders of the I4 muscles are drawn using thick black lines. Note that the radular stalk is entirely within the odontophore (A,D) and that the radula is open (G). The second column shows the radula/odontophore at the onset of retraction (start of t1) from frame 26 of sequence 7732-S3. The radular stalk is still entirely within the odontophore. We hypothesize that the presence of the stalk between the I4 muscles as they begin to compress together induces the I4 muscles to deform upwards and form a ridge (B,H), enhancing their ability to grasp food as they close. Note the shortening of the I7 muscles (E) relative to early protraction (D). If the I7 muscles contribute to holding the radular stalk between the I4 muscles, they could enhance the early phase of closing in this configuration. The third column shows the radula/odontophore during retraction (end of t1 period) from frame 34 of sequence 7732-S3. The radular stalk has moved maximally out of the odontophore, allowing the I4 muscles to close on one another as the radular surface rolls downwards. This induces the formation of the radular `pinch' (I), and also lengthens the I7 muscles (F), so that their contraction can pull the radular stalk upwards and separate the I4 muscles (i.e. the I7 muscles and the radular stalk can open the radular halves by changing their configuration from column 3 to column 1). Contraction of the I5 muscles can contribute to closing (E) by pulling the radular stalk out of the I4 muscles, and contraction of the I4 muscles can further push the radular stalk downwards, causing the radular halves to close as the odontophore changes from its column 2 to its column 3 configuration. However, relaxation of the I4 muscles and movement of the radular stalk into the odontophore, separating the I4 muscles and lengthening the I5 muscles, could allow a contraction of the I5 muscles to cause the I4 muscles to rotate outwards, so that I5 may enhance opening (changing the odontophore from its column 3 configuration to an open configuration; column 1 shows the odontophore after the peak opening of the radular halves).

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