The Horseshoe Crab (Limulus polyphemus)
Although horseshoe crabs look dangerous, they are not. And they
are really not crabs at all; they are distant relatives of the
spider and are probably descended from the ancient order Eurypterida.
Like the Atlantic moonsail the lightning
whelk the knobbed whelk, and the channeled whelk, they
feed on clams; they also include worms and other invertebrates
in their diet. The horseshoe crab places a clam near its mouth
in the center of its underside where its legs are attached and
grinds and crushes with the burr-like sections of the legs.
The first four of the five pairs of legs are used for walking,
while the last pair, located near the gills have leaf-like flaps
that are used for pushing. (Males can be distinguished by the
first pair of legs which are heavier than those of the female.
The spike-like tail serves as a rudder, and, if the crab is flipped upside down, it may bend its abdomen at the point where it joins the main shell (carapace) and dig into the sand with the tail to support itself while it turns over.
The horseshoe crab has two pairs of eyes. The first--visible in the upper photo--are compound, and the second pair, located in the middle of the top front side are simple.