pair of limbs" are not employed for walking, but exclusively for burrowing. These limbs are terminated by four long, stiff, oval, or leaf-shaped lobes, jointed at their base to the leg, and capable of being opened and closed in a four-radiate manner. When it wishes to burrow these two limbs are sometimes alternately and sometimes simultaneously thrust backwards below the carapace quite beyond the hinder edge of the shell, and in the act of thrusting the lobes or plates on each leg encounter the sand, the resistance or pressure of which causes them to open and fill with sand, a load of which at every thrusting operation is pushed away from under the crab and deposited outside the carapace. The four plates then close, and are withdrawn closed, previously to being opened and charged with another load of sand, and at the deposit of every load the whole animal sinks deeper in its bed, till it is hidden all except the eyes. The great overarching shield of the carapace again prevents one from seeing whether this excavating work is being aided by the fanning motion of the branchial false feet, but I think such a fanning is going on, as I have seen signs of sand being driven out as if urged by a current of water.
Mr Lloyd thus describes the use of the tail spine in locomotion:
The animal having climbed up a rock in the aquarium till it has got near the top of the tank (which in Hamburg contained a depth of 30 inches of water), and having assumed a vertical position, it leaves go its hold on the rock and allows itself to fall back into the water; but its downward fall is instantly checked, and the creature propelled upwards by the powerful flapping of its false branchial feet, and when the impetus given by these appendages ceases the animal again sinks down, but is prevented from falling prone on the floor of the tank by alighting on the tip of its tail spine. The moment this happens, and before the creature has lost its balance on the spine, the false feet make another flap and give another impulse upwards and forwards; all this time the position of the carapace is slanting, the top inclining downwards at an angle of about 45°, the hinder shell being at another angle, and the tail spine hanging down vertically, and so it progresses by a combination of flapping and hopping till it reaches the limits of its tank and sinks to the ground. The Limulus was fond of thus going about at night, generally remaining on the sand all day.
Another use made of the tail spine was as a lever by means of which it righted itself when it fell off a rock on its back. The spine is then bent, its point is planted in the sand so that it makes an acute angle with the carapace, which is then so far raised that some of the feet are enabled to grasp a projecting surface, and the crab then turns over.
