Price Rebecca M
Department of Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave., Chicago, Illinois 60637, USA.
Biol Bull. 2003 Dec;205(3):351-66. doi: 10.2307/1543298.
Malacologists often assume that ornamentation on snail shells is functional, and therefore adaptive. I conducted the first comprehensive test of the widely accepted hypothesis that columellar folds, a type of internal ornamentation, enhance the performance of the columellar muscle, which attaches the snail to its shell. Careful dissections of live, non-relaxed specimens reveal that the physical attachment between the columellar muscle and the columella is not restricted to a small, circular patch located deep within the shell. Instead, the attachment is long and narrow, extending approximately a full whorl along the length of the columella. I developed a novel technique for preparing three-dimensional reconstructions from photographs documenting the dissections. These reconstructions were then used to measure four parameters that describe the muscle: (1) the surface area of the physical attachment between the muscle and columella, (2) the total contact area between the muscle and the columella, (3) the depth of attachment, and (4) the length of attachment. None of these parameters differed significantly between species with and without folds. In light of the biomechanics of muscular hydrostats, values of the first parameter indicate that columellar folds probably do not guide the columellar muscle as the animal moves in and out of its shell. Values of the other parameters indicate that columellar folds neither increase an animal's ability to maneuver its shell nor facilitate deeper withdrawal. These results, and the fact that folds have evolved convergently several times, might indicate that folds are an easily evolvable solution to many functional problems, none of which are currently understood.
软体动物学家常常认为蜗牛壳上的纹饰具有功能性，因此具有适应性。我对一个被广泛接受的假说进行了首次全面测试，该假说认为柱状褶（一种内部纹饰）能增强柱状肌的性能，柱状肌将蜗牛与壳相连。对活体、未放松的标本进行仔细解剖后发现，柱状肌与小柱体之间的物理连接并不局限于壳内深处的一个小圆形区域。相反，这种连接是狭长的，沿着小柱体的长度延伸约一整圈。我开发了一种新技术，用于根据记录解剖过程的照片制作三维重建图。然后利用这些重建图测量描述该肌肉的四个参数：（1）肌肉与小柱体之间物理连接的表面积，（2）肌肉与小柱体之间的总接触面积，（3）连接深度，以及（4）连接长度。有褶和无褶的物种在这些参数上均无显著差异。根据肌肉流体静力学的生物力学原理，第一个参数的值表明，当动物进出壳时，柱状褶可能无法引导柱状肌。其他参数的值表明，柱状褶既不会增加动物操控壳的能力，也不会促进更深程度的缩进。这些结果，以及褶已经多次趋同进化的事实，可能表明褶是对许多功能问题的一种易于进化的解决方案，而目前对这些问题尚不清楚。
