Fig 1. Development of Sepia officinalis and localisation of main nervous structures (after [38], modified).

(A) Organogenesis. St18: Plane phase (Stage 14–18) Stage 18, aboral view; the embryo is “disc-shaped” on the surface of yolk. St20, St22: Extension phase (Stages 19–22). St20: Stage 20, aboral view; the shell sac is closed and the two funnel tube elements grow. St22: Stage 22, ventral view; the funnel tube is formed. St25, St28, St30: Growth phase (Stages 23–30), dorsal view. St25: Stage 25; the shell begins to form, eyes are clearly coloured. St28: Stage 28; skin is clearly coloured. St30: Stage 30, just before hatching. (B) Muscular system: the mantle, the two elements of the funnel (funnel pouches and funnel tube) and arms, essential for locomotion, are coloured. (C) Nervous system: at stage 16, nervous system is composed of sparse ganglia. Cerebroid ganglia (red) will constitute the dorsal supraoesophageal mass of the brain, pedal (yellow) and visceral (blue) ganglia will constitute the anterior/median and posterior (respectively) sub-oesophageal mass of the brain (depicted in stage 24 dorsal view). Optic ganglia (orange) will constitute optic lobes; with the brain, they constitute the central nervous system. Stellate ganglia (green), located symmetrically on the internal side of the mantle and brachial ganglia (one by arm, which develops into a cord inside the arm) belong to the peripheral nervous system. For more information about development see [53]) a1, a2, a3, a4, a5: arms 1 to 5; bg, brachial ganglia; cg, cerebroid ganglia; e: eye; g: gill; mo: mouth; og, optic ganglia; pg, pedal ganglia; s, shell; st, statocyst; sg, stellate ganglia; ss: shell sac; t: tentacle; vg, visceral ganglia; y: yolk. Orientation: A(nterior)–P(osterior), L(eft)–R(ight). Scale bar: 1 mm.