Fig. 3: Anatomy of juvenile C. magus venom apparatus.

Six days after the onset of metamorphosis, all components of the venom apparatus were present and fully differentiated. a Left lateral view of the venom apparatus reconstructed from stained histological sections. Doted lines indicate planes of section shown in c, d, with the black arrowheads indicating the regions of the radular sac and venom gland sectioned in b, e, respectively. b Transmission electron microscopy (TEM) revealed that the short arm of the radular sac (rss) had started accumulating chitinous radular teeth (numbered 1–3) (see Supplementary Fig. 2a). Scale bar = 5 μm. c Just posterior to the buccal cavity, the venom gland (vg) runs beneath the oesophagus (oe). Scale bar = 30 μm. d The blind end of the venom gland was encapsulated with the venom bulb (vb), which consisted of two layers of muscle fibres (1,2) separated by a thin layer of collagen (black arrowhead) enclosing a wide lumen (lu). Scale bar = 30 μm. e TEM of a cross-section through the proximal venom gland. The cytoplasm of the venom gland cells was filled with small, spherical secretory granules (see Supplementary Fig. 2b, c). The dissociation of epithelial cells from the basal lamina (white arrowhead) and rupture of their membrane indicate holocrine secretion of venom granules into the lumen (lu). Scale bars = 10 μm. bc buccal cavity, bt buccal tube, lu lumen, m mouth, n nucleus, oe oesophagus, pr proboscis, rsl long arm of radular sac, rss short arm of radular sac, vb venom bulb, vg venom gland. Four juveniles at 6 dps were sectioned, with one used for the 3D reconstruct and one used for TEM.