There were a great variety of molluscs; indeed, the whole shore was composed of shells.  We naturally thought that the shells were empty; but as we watched them, thousands of them began to move, each tenanted by a soldier-crab, and a whole army of them slowly advanced out of the sea and marched across the land, devouring all the insects they encountered in their progress.  Now and then two of them would stop and have a fight over a beetle or a spider, when perhaps a third would step up and carry off the cause of dispute.  We found the spiders' webs stretching in every direction between the bushes.  The spiders themselves were great, ugly, black fellows, very disagreeable to look at, and still more unpleasant when we found them crawling over our faces.