In order to verify whether the transverse-oriented structure formed from the progeny of the 2b-micromere is indeed a muscle, 48 to 52 h old larvae were stained with phalloidin and analyzed. The fluorescence of the main larval retractor, accessory larval retractor, pedal plexus, velum muscle ring, and the anlagen of the left and right adult shell muscles was clearly visible. In addition, an unknown muscle was detected (Fig. 4). The morphology of this muscle was exactly like that of the other muscles, although the phalloidin staining of this unknown muscle was weaker. This newly-discovered muscle is located between the two lateral shell walls and is U-shaped. The base of the “U” is located near the hinge of the operculum. The fluorescence of this muscle co-localized with the fluorescence of the transverse-oriented band of fibers observed after tracing the 2b-progeny with TMR-dextran.
Fig. 4. Stereo images and schematic drawings of 48 to 52 h old larvae stained with fluorescently-labeled phalloidin. ( A) Stereo image. Ventral view of a 56 h old post-torsional larva. Phalloidin staining. The unknown muscle is clearly visible (new). In the right side of the larva, the four fibers of the accessory larval retractor (alr) are visible. (B) Schematic drawing of A. (C) Stereo image. Animal view of a 48 h old post-torsional larva. Phalloidin staining. The unknown muscle (new) is located perpendicular to and ventrally of the main larval retractor. The four fibers of the accessory larval retractor, located in the right side of the larva, are indicated as well. (D) Schematic drawing of C. For legend see figure 1. Scale bar: 50 µm.