Hearts of invertebrates can be categorized according to the source of the electrical rhythmicity that underlies their beat. Rhythmic electrical activity can arise in the muscle itself (myogenic hearts) or in neurons that drive the heart muscle (neurogenic hearts). Most mollusks and some insects appear to have purely myogenic hearts; these hearts beat normally when isolated from neural inputs. Conversely, the hearts of the higher crustaceans and the xiphosuran Limulus are usually considered to be purely neurogenic: motor neurons impose their rhythmic electrical activity on heart muscle fibers by means of direct excitatory synapses. Without neural input, the heart ceases to beat. Other invertebrates, including gnathobdellid leeches and some insects, have hearts that can produce a myogenic beat but require rhythmic neural input to coordinate that beat and maintain the proper rate.