Table Extended Data 1. Synaptic input from bipolar tail neurons to motor neurons, identified by electron microscopy.

Postsynaptic motor neuron identity	Synapse partnership	Number of synapses	Total number of sections with synaptic profile
MN1 Left (A11.118)	BTN1-->MN1L	27	134
	BTN3-->MN1L	21	88
	Total	48	222
MN1 Right (A11.118)	BTN1-->MN1R	3	14
	BTN2-->MN1R	22	94
	BTN3-->MN1R	1	4
	BTN4-->MN1R	11	55
	Total	37	167
MN2 Left (A10.57)	BTN1-->MN2L	10	51
	BTN3-->MN2L	6	30
	Total	16	81
MN2 Right (A10.57)	BTN2-->MN2R	17	90
	BTN4-->MN2R	10	73
	Total	27	163
MN3 Left	BTN1-->MN3L	1	2
	Total	1	2
MN4 Left	BTN1-->MN4L	2	9
	Total	2	9
MN4 Right	BTN2-->MN4R	2	5
	BTN4-->MN4R	1	2
	Total	3	7
MN5 Left	BTN1-->MN5L	1	3
	Total	1	3
MN5 Right	BTN4-->MN5R	1	3
	Total	1	3

BTN, bipolar tail neuron. MN, motor neuron. Axons of BTN 1 and 3 lie on the left hand side of the embryo, and BTN 2 and 4 on the right. The axons are not traced to their somata to indicate which would be anterior and posterior.