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Regulation of spiral coiling in the terrestrial gastropod Sphincterochila: An experimental test of the road‐holding model

Hutchinson's ('89) road‐holding model states that spiral ornaments of the snail shell (keels and low‐curvature areas) dictate the growth path of the subsequent whorl, which in turn gives the signal for attachment of the next whorl. Experiments were performed with two species of the terrestrial snail Sphincterochila in order to test the role of the external keel in determining the correct coiling of successive turns. Experiments substituted a ridge made of silicone for the keel. This ridge ran either (1) abapical or (2) adapical of the original keel. In mode (1), subsequent growth continued by taking the false keel as the adapical limit of the whorl. In only very few instances of mode (2) did the whorls extend incipiently slightly adapical of the path of the original keel. Our results confirm that the keel is an important reference for the coiling strategy of the snail, although the keel itself probably does not constitute the reference, but rather the two flat ramps into which the keel divides the outer lip of the aperture. J. Morphol. 235:249–257, 1998. © 1998 Wiley‐Liss, Inc.