Limulus polyphemus

Horseshoe crabs possess five pairs of gills located just behind their appendages that allow them to breathe underwater, and can also allow them to breathe on land for short periods of time, provided the gills remain moist.

Although most arthropods have mandibles, the horseshoe crab is jawless. The mouth is located in the middle of the underside of the cepahalothorax with chelicerae located at each side of the mouth. In the female, the four large legs are all alike, and end in pincers. In the male, the first of the four large legs is modified, with a bulbous claw that serves to lock the male to the female while she deposits the eggs and he waits to fertilize them.

The Horseshoe Crab has blue blood, as it uses copper rather than iron as the base of its system.

Limulus has been extensively used in research into the physiology of vision. It has four compound eyes and each ommatidium feeds into a single nerve fibre. Furthermore the nerves are large and relatively accessible. This made it possible for electrophysiologists to record the nervous response to light stimulation easily, and to observe visual phenomena likelateral inhibition working at the cellular level. More recently, behavioral experiments have investigated the functions of visual perception in Limulus.

Limulus has two large compound eyes on the sides of its head, which have monochromatic vision. The individual ommatidia are complex, consisting of upwards of 300 cells; they number around a thousand, and are somewhat messily arranged, not falling into the ordered hexagonal pattern seen in more derived arthropods. An additional simple eye is positioned at the rear of each of these structures. In addition to these obvious structures, it also has two smaller ocelli situated in the middle-front of its carapace, which may superficially be mistaken for nostrils. A further simple eye is located beneath these, on the underside of the carapace. A further pair of simple eyes are positioned just in front of the mouth. The simple eyes are probably important during the embryonic or larval stages of the organism, and even unhatched embryos seem to be able to sense light levels from within their buried eggs. The less sensitive compound eyes, and the median ocelli, become the dominant sight organisms during adulthood.

The individual ommatidia of the compound eyes of Limulus Among other senses, they have a small sense organ which senses on the triangular area formed by the exoskeleton beneath the body near the ventral eyes.