Pallial system: (20) photograph of the ventral zone of the pulmonary roof, note the long kidney, with respect to the total length of the lung; (21) line drawing of the same region detailing the limits between kidney and ureter and the shallow veins crossing the pulmonary roof. Scale bar: 21 = 5mm. Abbreviations (k) kidney, (mc) mantle collar, (pv) pericardic vein, (r) rectum, (su) secondary ureter.