Pycnogonid visual neuropils studied with focused ion beam SEM technique. A, three-dimensional volume of low-resolution image stack; note sharp xz- and yz-projections due to almost perfect alignment of FIB-SEM. B, backscattered electron image of mesa at beginning of milling by FIB-SEM. Bar 100 μm. C–F, short consecutive image series at beginning of stack; note minor but visible structural change from slice number 80 (C) to slice number 83 (F). Bar 10 μm. Arrowhead, visual tract projecting through cell body rind; Th, thickening; VN1, visual neuropil 1. SEM, scanning electron microscopy.