. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 466 S. L. TAMM AND S. TAMM. Figure 3. DIG view of the surface of a peeled apart lip of Beroi- sp. in MgCI:/seawater. The lip edge is to the left; the aboral direction is to the right. The adhesive strip (bracket) runs parallel to, and a short distance aboral to. the macrociliary field (M). The surface of the adhesive zone is covered with numerous small vesicles or craters. Suhepithelial transverse muscle fibers (tm) run vertically and appear out-of-focus. The finer and more closely spaced longitudinal muscles (1m, cf. Fig. 5
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 466 S. L. TAMM AND S. TAMM. Figure 3. DIG view of the surface of a peeled apart lip of Beroi- sp. in MgCI:/seawater. The lip edge is to the left; the aboral direction is to the right. The adhesive strip (bracket) runs parallel to, and a short distance aboral to. the macrociliary field (M). The surface of the adhesive zone is covered with numerous small vesicles or craters. Suhepithelial transverse muscle fibers (tm) run vertically and appear out-of-focus. The finer and more closely spaced longitudinal muscles (1m, cf. Fig. 5) run horizontally and are visible in the area between the adhesive strip and the macrocilia. Scale bar, 30 ^m. The adhesive region in the mouth was located by ex- amining segments of adherent lips excised from Mg-re- laxed animals. We found that lip segments are fastened together along a narrow zone of the apposing stomodaeal walls (Fig. 2). The belt of tissue adhesion runs around the inside of the mouth, a short distance aboral to the band of macrocilia encircling the lip edge (Fig. 2). Therefore, mouth closure is not due to Velcro-like attachment of macrocilia on apposing lips, a possibility that initially had seemed likely to us. We next investigated the adhesive properties of closed lips. Adherent lip segments were pulled apart by forceps, starting at one end of the joined pair. Tension initially stretches the joined stomodaeal walls without separating the adhesive zone (Fig. 2A). Increased tension further draws out the two attached surfaces, which appear as mir- ror images of each other (Fig. 2B,C). Then the adhesive strips on either side separate at the site of greatest tension and the lips peel apart. This is followed by elastic recovery and flattening of the stretched stomodaeal walls. Darkfield microscopy of joined, stretched lips under tension shows that the zone of stomodaeal contact scatters more light, and thus appears brighter than the adjacent walls (Fig. 2C). T
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