(A) The larvae of Platynereis dumerilii swim by coordinating the beating of cilia on the surface of the body. The cilia are arranged into several bands known as the prototroch, metatroch and paratroch I, II and III. Scale bar, 50 μm. (B) Verasztó et al. mapped the connections between the three classes of motor neurons involved in swimming. The first class of neurons produce a neurotransmitter called acetylcholine and stop the cilia in the prototroch and the three bands of the paratroch beating during the cholinergic phase. These cholinergic neurons also connect to the crescent cell in the head, which has cilia that beat alternately to the cilia in the body. The second class of neurons produce a neurotransmitter called serotonin and make the cilia in the prototroch and paratroch beat faster during the serotonergic phase. The third class of neurons – known as catecholaminergic/peptidergic neurons – form a rhythmic pacemaker system that modulates cilia activity. Figure adapted from Verasztó et al., 2017.