The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1000 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. 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’.[4][5] The mouth is located in the center of the legs, whose bases have the same function as jaws and help grinding up food. The horseshoe crab has five pairs of legs for walking, swimming, and moving food into the mouth, each with a claw at the tip, except for the last pair. The long, straight, rigid tail can be used to flip the animal over if turned upside down, so a horseshoe crab with a broken tail is susceptible to desiccation or predation.

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