Branchial and systemic hearts from one O. insularis embryo. A Naef stage XVIII O. insularis embryo shown in a ventral view. For the first 10 seconds of the movie, only the branchial hearts are beating, which can be seen at lateral positions of the body. At this point, the embryo moves, and the systemic heart suddenly starts beating. The systemic heart is located between the two branchial hearts. At this point in development, the three hearts do not seem to beat synchronically.