Cross section through the ventral, branchiogenital region of the Lancelet Branchiostoma. A: Myomere, B: Notochord, C: Dorsal aorta, D: Epibranchial groove, E: Endostyle, F: Intestine, G: Pharyngeal cavity, H: Primary gill bar, I: Secondary gill bar, J: Lateral cilia, K: Ovary, L: Metapleural folds, M: Atrium.