Fig. 2. The parameters measured to quantify posture. (A) The width of the mantle was measured along line a, the width of the arms along line b. (B) Schematics of the cross-sectional view through plane 1 in A (sonogram of collar flaps). The left hand schematic represents an animal at rest, the right, an animal during the alternate response. The dorso-ventral height of the anterior mantle was measured along line c. The area of the anterior mantle cavity (water space) was stippled area d. (C) Schematics of the cross-sectional view through plane 2 in A (sonogram of the ventricle). As in B, the left hand schematic represents an animal at rest, the right, an animal during the alternate response. The dorso-ventral height of the mid-mantle was measured along line e. The area of the mid-mantle cavity was stippled area (f). avc, anterior vena cava; cf, collar flap; fu, funnel; m, mantle; all grey areas represent viscera; all white areas represent fluid-filled spaces.