(A) Schematic showing anteroposterior position (PAP), with 0 representing the snout and 1 the tip of the tail. (Bottom left) Schematic showing dorsoventral positions (PDV) in a hagfish in cross-section (see Materials and methods). (B) Linear density (λ) of each cell type for dorsal, lateral, and ventral regions along the hagfish (N=1 hagfish). Values are means ± SD. Dots represent data for each cross-section, with color representing PAP. The linear density of epidermal thread cells (ETCs) was relatively consistent as a function of circumferential position, with the mean value ranging between 17 and 27 mm–1. The other two cell types, small mucous cells and large mucous cells, had similarly uniform distributions. The small mucous cells exhibited the highest density (126–434 mm–1), and the large mucous cells had the lowest among the three (2–6 mm–1). (C) Among all three types of epidermal cells, only the large mucous cells exhibited a change in density with respect to the anteroposterior position. Based on linear regression models, we found no significant effect of anteroposterior position on the densities of ETCs and small mucous cells (p>0.1). However, in large mucous cells, we found a significant effect, with slope = –1.47 ± 0.30, p<0.001. (D) Area density () of small mucous cells and ETCs sampled from laser confocal images taken in en face view from two hagfishes at three different anteroposterior positions (N=2 samples per location). Values are means ± SD. Colored dots represent data for each sampled area. Data available in Figure 2—source data 1.