Fig. 5. Comparison of transilluminated and mid-sagittal magnetic resonance (MR) images of swallowing in Aplysia californica. (A-C) Mediolateral views of a transilluminated juvenile, reproduced from Fig. 3B, C and A (respectively) of Drushel et al. (1997). (D-F) Corresponding mid-sagittal MR images of an adult (sequence 5385-S1, frames 116, 122 and 128, respectively). (G-I) Schematic tracings of the anatomical structures in D-F. The left-hand column is rest, the centre column is peak protraction and the right-hand column is peak retraction. The characteristic ovoid (shape 2), round (shape 1) and (shape 3) buccal mass shapes (first defined in transilluminated images) are apparent in the MR images, as are the exact positions of internal anatomical structures. Note that some structures that appear opaque to transillumination do not appear dark in the MR images (D, dotted lines around elastic tissue joining the I1/I3 muscles to the lips; G, tissue labelled et), whereas parasagittal structures that cast a shadow in transillumination (e.g. the penis) are absent from the narrow mid-sagittal plane of the MR images. ba, buccal artery; w, wall of container holding the animal; e, esophagus; r, radula; rs, radular stalk. Transilluminated NTSC video frames are 33.3 ms. MR image frames are 250 ms.

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