RESUMEN
Objective: The purpose of this study was to demonstrate the structures of the third stage larva of Anisakis simplex in marine fish by using light microscope (LM) and transmission and transmission electron microscope (TEM). Methods: L3 Anisakis simplex were processed for conventional light and electron microscopic studies. Results: The body wall of the L3 A. simplex is composed of three layers, the cuticle, hypodermis and somatic muscular layer from the superficial to deep surface of the worm, respectively. The cuticle is thick, functions as a protective barrier and barring the antigenic molecules. Furthermore, TEM reveals that the cuticle is subdivided into cortical, middle and basal layers from outer to inner part of the cuticle. This layer is highly filamentous and the filaments arrange randomly in several directions. The hypodermis is a thin layer which functions as the cuticular productive layer. The lateral hypodermal cords are bilobed. The somatic muscular layer is composed of a single row of muscle cells, which lie along the long axis of the body. The gastrointestinal tract of the worm is a straight tube, lined with stratified epithelium and surrounded by the basal lamina. The intestinal epithelial cells contain various organelles, which its luminal surface presents numerous microvilli for absorption of nutrient molecules. Conclusion: The cross section of LM and TEM can be used to distinguish the nematode species, as the LM reveals the lateral cords of L3 A. simplex are bilobed structures which are different from other species.