The corpora pedunculata, or mushroom bodies, of the horseshoe crab, Limulus polyphemus, form a bulbous ventral hemisphere composed of two internal lobes that are highly branched like a caulifower. This organ is clothed with a deep layer of small association neurons called globuli or Kenyon cells. In an animal that is 50 mm in width, they number 3-7 X 10(6), a value that rises to about 1 X 10(8) in an adult (250 mm width). The neuropil of each corpus peduculatum converges from its peripheral lobules toward several major peduncles, which are in communication with the protocerebral neuropil by a narrow stalk containing about 5000 fibers in a 50 mm animal. The numberical relations suggest that presumptive second-order chemosensory fibers enter the corpora pedunculata and synapse divergently onto Kenyon cells. The axons of Kenyon cells, in turn, converge onto efferent fibers that leave through the stalk.