Anatomy of swim interneurons. Top: A diagram of the Melibe brain showing the location of swim interneurons I and II (SiI,SiII) on the right side of the brain. They also have counterparts on the left. The general locations of the left pedal swim motoneurons are shown in the left pedal. There are approximately 20 in each pedal primarily on the dorsal surface. Bottom: lucifer fills of the left SiII and right SiI. SiII is located in the middle of the pedal ganglion and it projects through the pedal-pedal connective to the opposite pedal ganglion. SiI is located in the cerebropleural ganglion and it projects to the ipsilateral pedal ganglion.