The HPT of the freshwater crayfish. (A) Localization of the HPT in crayfish underneath the carapace (the tissue covering the dorsal surface of the stomach and surrounded by a thin sheet of connective tissue). (B) The HPT can be isolated in one piece. Note the ophthalmic artery (AO, arrows) traversing the tissue in the anterior-posterior direction, bar = 2 mm. (C) A lobule of the HPT with a sharp apical end and diffuse distal region, bar = 20 μm. (D) BrdU incorporated into the HPT cells in vivo showing a high proliferation rate in the thin sheet of HPT cells (arrow on left) covering the stomach (arrow on right), bar = 100 μm.