258 THE HORSE FOOT CRAB. 
it never seeks the shallow waters, unless for the purpose of 
reproduction. It is emphatically a burrowing animal—living 
literally in the mud, into which it scoops or gouges its way 
with great facility. The anterior edge of its enormous 
cephalic shield is not unlike in form the sausage, or mince- 
meat knife of our kitchens (Pl. 3, Fig. 12). The upper 
shell of the animal is composed of three parts—the forward 
shield, which is greatly the larger, the posterior shield, and 
the long bayonet-shaped spine, or tail. In the burrowing 
operation the forward edge of the anterior shield is pressed 
downward, and shoved forward, the two shields being in- 
flected, and the sharp point of the tail presenting the ful- 
crum as it pierces the mud, while underneath the feet are 
incessantly active, scratching up and pushing out the earth 
on both sides. There is a singular economy of force in this 
excavating action, for the alternate doubling up or inflecting, 
and straightening out of the two carapaces, with the pushing 
purchase exerted by the tail, accomplish both digging and 
subterranean progression. Hence the King Crab is worthy 
to be called the Marine Mole. 
The Limulus is carnivorous. Its food is the soft nereids, 
or sea worms ; so that not only in its mode of burrowing for 
concealment, but also in its method of procuring food does 
it resemble that little burrowing mammal of the land. It is 
sometimes found held in a strange durance, with a limb en- 
trapped between the valves of the quahog, or round clam, 
( Venus mercenaria). It is a pitiful sight to behold—a galley 
slave with limb confined to ball and chain— "as far from 
help as limbo is from bliss." The explanation is easy. The 
quahog too is a burrower, and Limulus has seized the pro- 
jecting syphon of the molluse, which being suddenly with- 
drawn, the less agile claw is jerked between the valves, and 
the same are closed. This, of course, would effectually 
entrap the limb. But here occurs just this strange fact, that 
a lobster or a crab would not long be held in such durance, 
but would give their custodian leg-bail; that is, would cast 
