The entire body of the horseshoe crab is protected by a hard carapace. It has
2 compound lateral eyes, each composed of about 1,000 ommatidia, plus a pair of median eyes that are able to detect both visible light and ultraviolet light, a single endoparietal eye, and a pair of rudimentary lateral eyes on the top. The latter become functional just before the embryo hatches. Also, a pair of ventral eyes is located near the mouth, as well as a cluster of photoreceptors on the telson.
The horseshoe crab has 5 additional eyes on top of its shell. Despite having a relatively poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans',[5][6] and their eyes are a million times more sensitive to light at night than during the day.[7]