The epithelia covering the filament and lamellae
can be differentiated, obviously by location, but
also by thickness, blood circulation (arterioarterial
vs. arteriovenous, see Olson, 2002,
this issue), and cell type composition (Fig. 3). In
the hagfish, the thickness of the epithelium
decreases gradually as the divisions of the surface
are more complicated (up to sixth-order folding)
compared to the lamellae of all other fishes, which
represent only first-order folding of the filament
(Fig. 4).