Fig. 7
From: ‘Arm brains’ (axial nerves) of Jurassic coleoids and the evolution of coleoid neuroanatomy
Orthoslice made from a CT-data volume (left) and histological section (right) cutting through the arm bases and buccal mass of Spirula spirula (Linnaeus, 1758). The arm bases encircle the buccal mass. Each arm has longitudinal muscle fibres surrounding an indistinctive grey mass. The darker spot in the central or subcentral position represents the axial nerves. The ratio between arm diameter and nerve diameter varies between 6:1 in dorsal arms and 4:1 in ventral arms