Gill receptor arrays in the horseshoe crab (Limulus polyphemus).

The branchial warts on the endopodites of the gills are covered with goblet-shaped cuticular appendages, whose internal structure shows them to be chemoreceptors. The innervated goblets have a cuticular tubule that connects an external pore through their hollow interior with the epidermal sensillum. Associated sensory neurons give rise to small axons that pass through a synaptic plexus below the epidermis. The sense organs seem specialized for sampling the exhalent water current.