The central nervous system of the cypris larva of Balanus amphitrite consists of a brain and posterior ganglion. The neuropil of the brain includes protocerebral and deutocerebral divisions, with nerve roots from the protocerebrum extending to the eyes and frontal filaments, and nerve roots from the deutocerebrum extending to the first antennae (antennules) and cement glands. The neuropil of the posterior ganglion includes subesophageal and thoracic divisions, with nerve roots from subesophageal divisions extending to the gut, and nerve roots from each of the six thoracic divisions extending to their corresponding thoracic appendage. The antennular nerve is the major peripheral extension of the nervous system and is composed in part by afferent fibers that innervate setae on the antennules. The cyprid nervous system is small, containing fewer than 2000 neurons, but is well organized for coordinating a response to settlement cues.