Five larvae at G42 in both controls and NO3–N treated groups
were euthanized, and fixed with 4% paraformaldehyde. For gross
morphology examination, the larvae transferred into 70% ethanol,skin and interhyoideus muscle were removed to expose thyroid
gland under an Anatomical Microscope (Optec, China). Specimens
of each treatment were observed under a Zeiss Discovery V12
stereoscope equipped with Cannon 7D digital cameras.