. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 368 C. D. DREWES AND C. R. FOURTNER 2 mm Frame no. Elapsed time (ms) DARK. 33 433 466 . 533 Figure 7. Frame-by-frame videotape sequence of rapid shortening in an isolated tail fragment in re- sponse to an abrupt decrease in light intensity. Frame 0 shows the tail fragment before onset of the stimulus. Rapid shortening occurred between frames 13 to 16. cells (Fig. 8C); (2) a basal nucleus; and (3) basal projec- tions into a radial nerve plexus directly beneath the epi- thelial layer. Thin sections through the body wall
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 368 C. D. DREWES AND C. R. FOURTNER 2 mm Frame no. Elapsed time (ms) DARK. 33 433 466 . 533 Figure 7. Frame-by-frame videotape sequence of rapid shortening in an isolated tail fragment in re- sponse to an abrupt decrease in light intensity. Frame 0 shows the tail fragment before onset of the stimulus. Rapid shortening occurred between frames 13 to 16. cells (Fig. 8C); (2) a basal nucleus; and (3) basal projec- tions into a radial nerve plexus directly beneath the epi- thelial layer. Thin sections through the body wall also provided de- tails regarding the ciliated epithelial cells (Fig. 8D, E). Both uniciliate and multiciliate cells were evident. The multiciliate cells generally have four to six cilia with a corresponding number of basal bodies. In the uniciliate cells, the cilium arises from a pit in the apical region of the cell and is surrounded by 10 microvillar projections, each containing an abundance of microfilaments and an electron-dense region adjacent to the ciliary process. The microvilli (approximately ^m in cross-sectional area and /urn in width) are also interconnected via an or- ganized, electron-dense material (Fig. 8E). These two cil- iated cell types are nearly identical to the proposed epi- thelial mechanoreceptors described in the anterior seg- ments of earthworms (Knapp and Mill, 1971; Mill, 1982) and an aquatic oligochaete, Rhynchelmis linio- sella (Moritz and Storch. 1971). the latter representing the same family as Lnmbriculus ( Lumbriculidae). Discussion Adaptive significance and timing oj the shadow reflex Stimulus modalities that elicit rapid tail withdrawal in L. variegatus include touch, moving shadow, and abrupt decrease in light intensity. These modalities, as well as the specific escape movements they elicit, appear well matched to the worm's specific lifestyle and habitat. The worms are especially abundant in the shallow margins of ponds where
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
